Added a basic validator for CSPPlugin. See #42.

A parser and a validotr were supposed to be written to solve #42. But
the parser I was building using pyparsing doesn't work very well and I
also have a difficult time actually validating through that parser.
Instead of wasting more time building a parser, I have decided to
implement a basic validator plus a url regex within the source code.
This one works for the general purpose.

I will continue my work started in
https://github.com/yeukhon/csp-validator.