Publish Pull Requests

[ghost]

We discussed this at the offsite, and I just got another request from mmeister. I think gaktive & mnapp would highly appreciate as well though.

• [ ] ideally, check which parts were changed re: compared branch and only build the affected parts/books
• [ ] write a comment with a link to the generated output into the PR
• [ ] make sure we are automatically cleaning up when the branch is gone

[ghost]

how this might work: from #24, which is a dupe i opened:

1. git diff --summary master..feature/branch
2. fetch all IDs from those files
   • AsciiDoc file may not have enough IDs
3. build unprofiled bigfiles from all MAIN= values in the repo's DC files
4. check the list of IDs gathered in (2) against their parent book/article IDs
5. build all affected books/articles (but not sets if we can avoid it) from their DCs
   • specifically highlight profiled parts via stylesheet
6. Publish at github.com/susedoc/doc-[product]/PR[PR_ID]/...
7. Add PR comment with a link to the index.html file https://damien.pobel.fr/post/github-api-from-travisci/