feat: add support for async compiling

[gijswijs]

jstransformer-nunjucks doesn't support asynchronous rendering, but nunjucks does, as explained in issue #35.

The basic jstransformer library looks for compileAsync, just before it defaults to the compile method as a final resort when compileFileAsync is called. This PR implement compileAsync

Nunjucks uses callbacks for asynchronous support, but jstransformer expects promises. Therefor the render function is denodified, which means that the render functions is wrapped in a promise, and the callback resolves the promise.

[ci-reporter]

The build is failing

✨ Good work on this PR so far! ✨ Unfortunately, the Travis CI build is failing as of c58982a. Here's the output:

npm test

> [email protected] test /home/travis/build/jstransformers/jstransformer-nunjucks
> test-jstransformer

 • nunjucks
   ✓ transform has an output format (0ms)
   ✓ transform has input formats (0ms)
   • basic
     ✓ nunjucks.compile() (5ms)
     ✗ nunjucks.compileAsync() (1ms)

       TypeError: template(...).trim is not a function
           at checkFunctionOutput (/home/travis/build/jstransformers/jstransformer-nunjucks/node_modules/test-jstransformer/index.js:88:38)
           at transform.compileAsync.then.template (/home/travis/build/jstransformers/jstransformer-nunjucks/node_modules/test-jstransformer/index.js:117:11)
           at tryCallOne (/home/travis/build/jstransformers/jstransformer-nunjucks/node_modules/promise/lib/core.js:37:12)
           at /home/travis/build/jstransformers/jstransformer-nunjucks/node_modules/promise/lib/core.js:123:15
           at flush (/home/travis/build/jstransformers/jstransformer-nunjucks/node_modules/asap/raw.js:50:29)
           at _combinedTickCallback (internal/process/next_tick.js:132:7)
           at process._tickCallback (internal/process/next_tick.js:181:9)

Total duration 15ms
tests failed

I'm sure you can fix it! If you need help, don't hesitate to ask a maintainer of the project!

---

Failed build for d427f8f

npm test

> [email protected] test /home/travis/build/jstransformers/jstransformer-nunjucks
> test-jstransformer

 • nunjucks
   ✓ transform has an output format (0ms)
   ✓ transform has input formats (0ms)
   • basic
     ✓ nunjucks.compile() (6ms)
     ✗ nunjucks.compileAsync() (1ms)

       TypeError: transform.compileAsync(...).then is not a function
           at TestCase.test [as fn] (/home/travis/build/jstransformers/jstransformer-nunjucks/node_modules/test-jstransformer/index.js:116:55)
           at /home/travis/build/jstransformers/jstransformer-nunjucks/node_modules/testit/lib/suite.js:74:29
           at tryCallOne (/home/travis/build/jstransformers/jstransformer-nunjucks/node_modules/promise/lib/core.js:37:12)
           at /home/travis/build/jstransformers/jstransformer-nunjucks/node_modules/promise/lib/core.js:123:15
           at flush (/home/travis/build/jstransformers/jstransformer-nunjucks/node_modules/asap/raw.js:50:29)
           at _combinedTickCallback (internal/process/next_tick.js:132:7)
           at process._tickCallback (internal/process/next_tick.js:181:9)

Total duration 15ms
tests failed

---

This comment was automagically generated by ci-reporter. If you see a problem, open an issue here.

[gijswijs]

I think the tests are wrong. The test assumes that template.fn(locals) returns a string. Hence the test
fails when that functions returns a promise, because template.fn(locals).trim() returns an error. By
wrapping it in a Promise.resolve() both the situations where template.fn(locals) returns a string or a Promise are covered. This is also how the jstranformer library works, so the tests now better mimick the behavior of the actual library. The test-jstransformer PR also adds a switch for async testing.

I also made a PR for the tests: jstransformers/test-jstransformer#50
If that one gets merged, we can go ahead and see if this passes.