This commit introduces two new types into the API: Hook, and Filter.
Both `Hook` and `Filter` are abstractions built on Git hooks and filters
respectively. Each type knows how to install and uninstall itself. These
processes are utilized by the setup method in the `lfs` package, and the
appropriate calls have been updated in the init and uninit commands.
These abstractions were introduced to make adding/removing required filters and
hooks easier for future projects, including the migration away from the smudge
filter.
Eventually it seems appropriate to move both new types into the `git` package,
as opposed to the `lfs` package. At the time of writing this commit, there is
some coupling against state defined in the `lfs` package (i.e., whether or not
we're currently in a git repo, the local working directory, etc). At somepoint
it would be nice to remove that coupling and move these new types into the
`git` package.
