Teach tmlanguage.js to process multiple .tmLanguage files at once

You can now pass more than one `.tmLanguage` file to `tmlanguage.js` and it will process each one in turn. This can vastly speed up the processing of multiple files as you don't have to pay the node/V8 startup cost for each file.

Note that the script's "dev mode" is now controlled by a `--dev` flag rather than an unnamed second argument.

/cc @gjtorikian