Add support for building without supplying a sln/proj.

This allows the normal msbuild behaviour, which looks in the current directory for a single item to build (see #41).

I've done some basic manual testing of this; and it seems to behave behave as expected; but I'm not sure if there might be any knock-on effects. I noticed there was a test.ps1 in the folder; but it's not clear to me how it works (nor does it look like it's testing much).

If you think this breaks anything. let me know and I'll investigate.
We should leave validation for the debugMode parameter set.,In that case the MSBuild APIs are called so a value is required.