More featureful base commit_status

We use buildbot for our testing infrastructure. It would be nice if we could provide things like a custom stage string, a retry_url, trigger warnings, set tags, and set things like :allow_failure for our builders.

I'm thinking of these additional optional parameters to the POST for the commit status API endpoint:

• allow_failure (bool)
• tags (list of strings)
• show_warning (bool)
• cancel_url (string)
• retry_url (string)
• download_url (string)

A field for an additional link (say, to documentation hosted for the build or some other bit specific to whatever CI setup there is) would be nice as well.