Update the tag-generation command

While testing #2902266: Support for Universal Ctags I noticed a few things that could be improved about drush vimrc-tag-gen (short formdrush tags):

1. Add an option to limit the languages parsed, with sensible default value. (This is especially important when used with Universal ctags after #2902266: Support for Universal Ctags.)
2. Add an example to the help text describing how we generate the tags files distributed with this project.
3. Make sure that sites/*/files/* gets excluded.