issues/commit to be links, not spans

Created by: trevorah

The problems that I want to solve are these:

• you cant write markdown for a link to a commit that has the text of a commit message but is also decorated like a commit. I.e you cant write the "pushed commits to github" commits in the activity feed in markdown.
• we're converting all our span issues to links on the client, which is just silly.

Id also like to turn all @mentions into links as well (wdyt @lerouxb?) Id like to also add a githubContext option to the processor, so that we stop decorating #1 as an issue if you are not in a repo room or sub channel. That make our issue decoration much more reliable.

opinions please @suprememoocow @lerouxb

(btw there are examples committed as tests)