Added 'from_code' and 'language' options to pass through to xgettext

By default, xgettext interprets input files as ASCII, so in order to use UTF-8 encoded input files, you need to be able to set the `--from-code` option (exposed as `from_code`). In some cases, it is also useful to be able to specify an input language (using `language`, passed through to `--language`) for xgettext.
Bumped package version
Allow use of xgettext's `--join-existing` option via `join` task option
Documented new option
Bumped package version
Renamed `join` option to `overwrite`, as it's more descriptive
Updated documentation to reflect renaming of `join` option to `overwrite`
Merge pull request #1 from robinnorth/master

Expose more xgettext options to the Grunt task