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.
Merge pull request #2264 from aroben/convert-many-tmlanguages

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