Support Commonmarker v1 api

[unasuke]

What

retry of rtomayko/tilt#396

The Commonmarker gem released v1 incluging breaking api changes.
https://github.com/gjtorikian/commonmarker/releases/tag/v1.0.0

These changes supports the new commonmarker's api.

To support commonmaker's breaking api changes, tilt gem will release new version including incompatible api change also.
For notice to tilt gem user, is it better to check commonmarker gem's version?

... and support both of CommonMaker (0.x) and Commonmarker (1.0.x)

[jeremyevans]

Thanks for working on this. This is a great start. Please see requested changes. Also, I've enabled CI so you can see where the CI failures are.

[unasuke]

I made some commits above older changesets for review. I think it's better to squash before it merges.

[jeremyevans]

Looks good, thank you for making the changes. I'll squash merge this assuming CI passes.

[jeremyevans]

Looks like a couple spec fixes are needed to pass on Ruby 2.1 and 2.0. Just switch to a regular heredoc (<< instead of <<~) and do not indent, that should fix it.

[unasuke]

Fixed at 290541d !

[jeremyevans]

Thank you very much for your work on this!

[unasuke]

@jeremyevans Thank you too, I'm glad if rtomayko/tilt#395 is merged also.

[jeremyevans]

The gem homepage was fixed. I agree that it would be best the old repository README was updated, but I have no control over that I would not expect it at this point.