Expose more xgettext options to the Grunt task

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.

Also added the option to specify whether to overwrite or append to an existing `.pot` file.