Use parse_args to ensure default options are set

When unsetting option values the key was getting removed from
option_value in wp_options, blowing up code that read from $options.
Using wp_parse_args will ensure those missing values get the default.
This only fixes errors in publish_to_discourse, there are other places
where this is still a problem.
Merge pull request #61 from wdbetts/master

Use parse_args to ensure default options are set