Prevent double-sourcing the plugin.

Improve startup performance and be a good Vim plugin citizen.

To ensure these commands are only created for Rails locale buffers, use
an `autocmd` that matches any locale YAML files in the Rails
`config/locales/` directory or subdirectories.

Make this a regular plugin instead of a filetype plugin. Rails locale
files are YAML files, but we don't want these commands to be available
for all YAML files, only those that match the path for Rails locales.
We won't be able to do the path match and filetype separation with an
ftplugin, so instead we'll set up the commands to be defined by
`autocmd`s that match the Rails locale path.