Support more configuration options

[swissspidy]

Previously: #12.

We likely need to support more if not all options that can be passed to wp i18n make-pot.

Note: Instead of putting them all together with sprintf() and exec() it's probably easier to use something like https://github.com/mikehaertl/php-shellcommand. We can then just iterate through an args array and add them all to the command object.