NEW: Add frozen versions of a plugin by its release tag

NEW: Frozen version is based on a release tag
BRAT will monitor and update plugins as they are updated in their GitHub repositories. However, if you want to freeze the version of a plugin, you can do so by using the command "BRAT: Add a beta plugin with frozen version based on a release tag". This will install the plugin with the version you specify, but not update it when other plugins are updated.

Thanks to the good work of @fyears for this feature