Docs auto-push to gh-pages branch

[tivac]

This modifies the existing travis script and changes the flow some so that docs will generate & auto push to the gh-pages branch when builds succeed on master.

Was going to use travis-scripts/update-branch since it would do this a bit more cleanly but it has no option to preserve files on the target branch which we need for archived versions. So gh-pages and some flopping around it is.

TODO once merged

• Verify that gh-pages branch looks correct
• Copy archived versions from lhorie/mithril into the correct location in the gh-pages branch
• Set up Github Pages to use gh-pages branch (Only @lhorie can change repo settings)
• Tweak mithril.js.org to point to Github pages (I don't know how to do that)
• Rejoice at never manually pushing site updates again

Caveat I would love to have docs changes to next auto-generate docs for another site, something like mithril-next.js.org, but that doesn't seem easy currently. Closest I can get so far is to spend some time figuring out how to get it to build changes on next into some sort of ./next folder in the gh-pages branch.

@lhorie verbally (textually?) approved this work in gitter

[pygy]

What are the plans for the Gandalf builds?

[pygy]

@tivac semi-related: If you ever get around automating the change log, could you also give a look at the GH API for doing releases (#1676 (comment))?

[tivac]

What are the plans for the Gandalf builds?

The $(npm bin)/commit-changes line will commit the changes generated by before_script: npm run build when on the next branch, so there should be no changes there.

If you ever get around automating the change log, could you also give a look at the GH API for doing releases

I would love to do this, but it's more complicated. I've been enjoying using @studio/changes on my personal projects for changelog generation.

Something like release and a commit hook via husky and validate-commit-msg to ensure that commit messages describe the change level correctly might get it done. A bit wary of that level of tooling though since @lhorie has been reluctant in the past to go too heavy down that road.