Add known parameters to CakeOptions

[patriksvensson]

Before adding module support, there was a few hacks to get the mono and roslyn experimental compilers configurable via command line flags as well as a debug flag for debugging. We used horrible things like _options.Arguments.ContainsKey("mono") to check for the existance of a specific flag which means that passing --mono=false will fail.

We should add explicit properties to the CakeOptions class and include those in the ArgumentParser for the following things:

• Mono
• Experimental