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.