Add unit test for single-sync feature
Update documentation for single-sync operation
Pass arguments to the sync command along
Accept an additional files parameter in sync
Restrict the sync files to what user specified

If the user doesn't specify a set of files to sync then the entire set of
dotfiles is assumed.  There is an obvious bug here, as user-passed strings are
not yet converted to dotfile objects.
Implement single-sync for specified files

This commit allows the user to sync only one (or a subset) of the available
dotfiles in the repository.