Add the ability to compile the code directly to an assembly in the command line tool

[mwadams]

We would like to be able to emit an assembly from the command line tool in addition to the raw C# code. This might be a friendlier path for non-C# developers.

We already have everything we need to be able to do this (specifically the map-file support which allows us to discover what we emitted and pass it in to the Roslyn compiler).

But we may discover we are just building a C# compiler tool front end, in which case this is a waste of time and it would better left to the end user.