Add support for attributes in webL10n.get, and corrected behavior of the fallback argument

Before this PR, it was impossible to programatically get the value of an localized attribute. I checked the [implementation of how attributes in the properties file are parsed](https://github.com/fabi1cazenave/webL10n/blob/b18c753c6fe81e733d86660d421ace7a7880e90f/l10n.js#L264-L268), and derived a method that recognized attributes, if specified.

Read commit message of dd0703a662a623e550317e3437f6850796606e9c for an explanation of the second change set. In a nutshell:

- Initial implementation: `webL10n.get('non_existent', null, 'fallback') === 'fallback'`
- Later implementation: `webL10n.get('non_existent', null, 'fallback') === '';` (expected 'fallback')
- My commit = back to initial implementation