Add .command as a Shell file extension

This pull request adds `.command` as a Shell file extension as requested in #966.
Many undetected Bash files can be found on GitHub:
https://github.com/search?utf8=%E2%9C%93&q=extension%3Acommand+echo&type=Code&ref=searchresults

EDIT: [Travis build](https://travis-ci.org/github/linguist/builds/38660201)