new mod linker

New mod linker that should remind you a bit of `git`.

Subcommands:

  list - List all the acceptable mods you can install.

  list <mod> - List all the acceptable versions of an acceptable mod.

  add <mod> - Add the latest version of a mod to the correct place in
  your instance.  Requires a ``LATEST'' file to work properly.

  add <mod> <version> - Add a specific version of a mod to the correct
  place in your instance.

  rm <mod> - Remove a mod from your instance.

Relevant environment variables:

  MODROOT - Where your mods are being stored.  This defaults to
  "../../mods".  Can be either a relative or absolute path.

  MCVERSION - The version of Minecraft if the file ``VERSION'' does
  not exist.  (Leaving out the version entirely is a deprecated
  behavior that I personally have not moved my instances off yet, so
  it's a bit of compatibility for me.)
be more restrictive in deletions

Be more restrictive when looking for files to delete.  Now you should
no longer delete your configuration file if it is named the same as
the mod when updating.

Oops.