add exclude_tag and exclude_category

[JulianKingman]

Solves #44
Needs to be tested, but it should work.
I'll test it tomorrow

[JulianKingman]

Tested it, it works

[JulianKingman]

@erwstout ready for merging

[stevengrimaldo]

@JulianKingman I'm unable to get the exclude_categories to work right when doing the following:

exclude_categories: '5, 27'

Example usage: https://dev.webapi.salata.com/wp-json/better-rest-endpoints/v1/posts?exclude_categories=5,27

Please help, thank you.