CRM-16243 - Extension API - Manage extensions by path

CRM-16243 - Extension API - Manage extensions by path

This allows one to enable or disable a series of extensions by path.

This should be useful, for example, when integrating with `composer` or
`drush make`. Without any knowledge of the specific extensions
involved, one might:

 * If you download a bunch of extensions to a common dir (e.g.
   composer's `vendor/`) and need to enable them all, then run
   `cv api extension.install path=$PWD/vendor/*` (circa `post-install-cmd`)
 * If you're deleting a specific directory (e.g. via composer's
   `uninstall`), then remove it gracefully from the DB by running
   `cv api extension.disable path=$PKGDIR` (circa `pre-package-uninstall`)