Support implicit object creation syntax and an attribute's constructors

Add support for requiring named arguments on

• the implicit object creation syntax
• an attribute's constructors

Support constructors

Jeff Ward (@fuzzybinary) contributed support for constructors.

Add support for [RequireNamedArgs] on extension methods

@kkkmail investigated and contributed code necessary to support [RequireNamedArgs] on extension methods.
See the corresponding issue for details.

Look for method definitions marked with a `[RequireNamedArgs]` attribute instead of a special comment

Look for method definitions marked with a [RequireNamedArgs] attribute instead of a special comment

With the exception of XML Documentation Comments, C# comments don't get emitted into the assembly's metadata.
As a result if a method marked // [RequireNamedArgs] from AssemblyA is invoked with positional arguments from AssemblyB
the analyzer doesn't emit any diagnostics for such an invocation. But it should!
Using an attribute seems to be the most reliable approach with regards to the above.

Initial release

0.0.1

Update README.md