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.
The problem is the line numbers in the fixtures can change a lot as tests change. My thinking was we'd have a single fixture for the line numbers tests that we'd be careful when modifying. But if you think churn won't be an issue then having everything exercise the line number extraction make sense.