Underscore extractor does not properly handle escaped characters

[amangano-edx]

The underscore template extractor does not seem to be able to properly parse strings that contain escaped characters. Such strings end up being written to the .pot file with extra escape characters.

Consider the following example:

An underscore template containing the following lines:

<%- gettext('foo\\bar') %>
<%- gettext('Bob\'s house') %>

Produces the following output in the .pot file generated by Babel:

msgid "foo\\\\bar"
msgstr ""

msgid "Bob\\'s house"
msgstr ""

This does not seem to be correct.

Please let me know if there's anything I can do to help fix this issue.

Thanks!

[amangano-edx]

Would passing the strings through https://github.com/python-babel/babel/blob/master/babel/messages/jslexer.py#L87 at some point resolve this issue?