Fix import command parameters

Apparently, the Commander gem treats options as boolean values
when one does not indicate that a certain options is followed
by a string. This led to the jekyll-import gem only receiving
booleans instead of dbname/user/pwd...

**Note:** Haven't added a test for this, because I wasn't quite sure how I should go about writing a test for it. If someone would provide me with a pointer, I'd be happy to add a test as well.