post a comment after building test table

[tdhock]

after building the plotly test table, the github API could be used to post a comment to the relevant PR

https://developer.github.com/v3/issues/comments/#create-a-comment

this could save developer time (less copy-pasting test table links), at the risk of overloading PR comments with lots of automatically generated links to the test tables.

[cpsievert]

I'd be even more interested in having the bot post a notification when the travis build is done. I often find myself returning to the PR page only to see that stupid yellow dot 😢

I suppose it wouldn't be too hard to provide links to tests added/removed as well