implement the exact opposite of the inverted match

[exaexa]

This implements {{?something}}...{{/something}} syntax, which works as an exact opposite of {{^something}}...{{/something}}, that is, fires precisely if the something does not evaluate to a falsey (empty, null) value. This allows implementing simple and exact if/else constructions, which is actually quite hard to achieve properly with original mustache. (I guess that lack of these is one of the ground reasons why e.g. handlebars.js exist.)

This is actually not in mustache spec but given the minor size of the extension and some other tiny improvements already in this repo, I guess it could be viable to get merged. (At the bare minimum I wouldn't need to keep the repo forked... :D)

Thanks for maintaining the library!

[exaexa]

PS I see that I actually added a few changes that might not be necessary (the 2 removed imports). I guess I hit some issues with those, unfortunately it is now over a month since I did this change, and I didn't write down the actual reason. If these are required for compatibility reasons, please ping me, I'll try to dig deeper and see why I actually had to remove these.

PPS. I just realize the tests are missing :D I'll try to write some if this looks OK for merging.