Support pointing mddocs at a solution or project file

[SonofNun15]

Would be really useful to be able to point the mddocs command line tool at a solution file or project file and have it build and then look up the executable instead of having to path to the built assembly.

Alternatively, the MSBuild tooling might eliminate the need for this if it could handle rendering multiple projects into the same root documentation folder.

Curious to hear your thoughts! Thanks!