Fixes #53 -- bug due false assumption that a version follows semver.

[sevalsixarci]

Hi, as title this fix #53

The bug occurs when a package (on GitHub) has a release tag with a name that is not a version number.
You can experience this with the package arturo (https://nimble.directory/pkg/arturo), which has an unexpected tag "v0.9.82-pre2" as you can see in the stack trace [...] invalid integer: 82-pre2.

The fix addresses this issue by disregarding the tag name and instead utilizing the date associated with the tag.

I also point out that since this code only works for Github (as before) it would fit better inside github.nim.

[sevalsixarci]

Ciao Federico,
I was wondering: your code expects the version as git tag that while (if annoted) has a date - as per my observation and proposed solution - doesn't mean that a newer tag is a newer version so this fix is incorrect.

Also the specific case presented as an example is actually following semver.

So I think we should discard this with another patch that correctly parses the version name. The second patch is based on semver package by @euantorano that I vetted having a stable working API and a compatible license.

Let me know your opinion.