Simplify main exported function.

Simplifies the variable argument checking and uses `object-assign` to assign defaults in a cleaner fashion. :smiley:
I made those in my master branch, but waiting for the other PRs to be sorted first :)

https://github.com/XhmikosR/uncss/commit/0711cefb6cfac608f258d11c221e21e95b319a00