Implement /NoWarn functionality

[jeffkl]

You cannot suppress all warnings, if you specify empty /warnasmessage or /nowarn, you get an error message.
Warnings are mutated into just a message.
Ignore the first commit which is the implementation of /WarnAsError

Closes #68
Closes #47
Closes #910

So this:

MyProject.proj(3,18): warning MSB4130: The condition "true or true and true" may have been evaluated incorrectly 
in an earlier version of MSBuild. Please verify that the order of the AND and OR clauses is written as intended. To 
avoid this warning, add parentheses to make the evaluation order explicit.

Becomes this in with Verbosity=Detailed

MyProject.proj(3,18): message MSB4130: The condition "true or true and true" may have been evaluated incorrectly
in an earlier version of MSBuild. Please verify that the order of the AND and OR clauses is written as intended. To
avoid this warning, add parentheses to make the evaluation order explicit.

Here is the help message for review:

  /warnasmessage[:code[;code2]]
                     List of warning codes to treats as low importance
                     messages.  Use a semicolon or a comma to separate
                     multiple warning codes.
                     (Short form: /nowarn[:c;[c2]])

                     Example:
                       /warnasmessage:MSB3026

[jeffkl]

@dotnet-bot test this please