categories and slug support

[len-ro]

hello,

While migrating my wordpress site to hugo my first try was to use this script. However the tags and categories were mixed. I added support to see if a wp category is a _posttag or category. I also used the first category as a slug in order to be able to restore my previous permalinks ( /category/post_name)

Thanks.

[palaniraja]

Thank you for the contribution, I am not sure if this is something everyone wants since it is not behind a command line option/flag, I'll update the readme to refer to this commit if anyone interested with similar expectation.