Factor out comment- and commit-text filters to make them pluggable

[cespare]

We should make comments and commit text filters (e.g. sha -> link replacement) automatically run any filters it finds in one location, and make it easy to write these filters. This will be especially useful when Barkeep is used outside of Ooyala, and introducing this will open the door for us to add some Ooyala-specific filters without putting them directly into Barkeep.

Once we've got this, here are some really simple filters that would be great to have:

• Convert APP-XXX to jira links
• Convert an image link (![alt][link] syntax) to a be a link to the image as well
• Make @username link to profile pages?

[philc]

All great suggestions. Good idea.