Extract line numbers from JavaScript

Esprima also supports column numbers, which would be pretty trivial to add to this code, but I've never seen that done in PO files. Let me know if that's something you'd want.

If you consider the generated POT file to be part of angular-gettext-tools's API, then this change requires a major version bump. Or you can turn off line numbers by default, but I think they're useful and should be on.
:+1: