Add `setting.Annex.Enabled`

[kousu]

Similar to server.LFS_START_SERVER, add a setting to toggle git-annex support on or off.

(Apparently that gets translated to setting.LFS.StartServer somehow, which is confusing because it's not under the LFS section but we'll figure it out.)

Currently this would control whether 'git-annex-shell' is enabled and whether the git-annex http support (#6) is enabled. Eventually this should control things like rendering web previews from annex or not.

[kousu]

Fixed by #18