Add analyzer for TheoryAttribute usage

[mikkelbu]

The analyzer should verify:

• That tests are only marked with Theory if they accept one or more arguments
• For each argument in the method one or more of the following sentences should be true:
  • The argument is either a boolean or an enum. Or a nullable variant thereof.
  • The argument is specified using a DatapointAttribute.
  • The argument is specified using a DatapointSourceAttribute.

See https://github.com/nunit/docs/wiki/Theory-Attribute and related pages for more info.