Wordpress mixin

# Work in progess
I created a Wordpress mixin to abstract some common wordpress methods. I also changed 2 modules to use the new mixin to see if it is working.
Would be great to get first feedback on it.

PS: Regexes and logic taken from https://github.com/wpscanteam/wpscan

# TODO:
- [ ] add rspecs

## TODO for later versions:
- [ ] update the other wordpress modules to use the new mixin
- [ ] add @CharlieEriksen methods from https://github.com/rapid7/metasploit-framework/pull/1301