Teach tmtheme.js how to convert a non-built-in theme

You can now pass a .tmTheme file on the command line to have it converted.
Make tmtheme.js handle elements with no settings

We just ignore them now rather than throwing an exception.
Pretty-print CSS even when creating a new file
Merge pull request #2247 from aroben/custom-tmtheme

Teach tmtheme.js how to convert a non-built-in theme