Bitbucket server support

Love this extension and hackmd!

The extension works great for me on github and gitlab, the last mile for me will be some of my projects that are in an on-prem bitbucket server instance, so I figured I'd suggest it.

Thanks for the awesome tools!

Start by examining how the extension currently supports GitHub and GitLab, then determine how on-premises Bitbucket Server support would fit. The issue names no files or tests; it is complete when the extension supports the requested Bitbucket Server projects alongside the existing providers.

索引モデルが issue の本文から書いたものです。