Add mechanism to gather and update bashable

[docwhat]

The user story is roughly this:

1. Doc has a project that uses bash for something. Doc wants to add bashables.
2. Doc runs an app and selects what parts are wanted.
3. The app downloads them from GitHub, bundles them up, and makes them ready to be committed into the project.

...

1. At some point in the future, Doc wants to update bashables.
2. Doc run an app.
3. The app tells him if any parts have changed significantly, and shows what will be updated.
4. Doc approves, and the bashable bundle is updated.