Add an ExplicitProject constructor, which reads options from a file

[alang9]

In the absence of a cabal config, cabal sandbox, or stack config, a ghc-mod.options file allows us to define a project root, and options to pass to ghc. This is useful for projects with custom build systems.

[DanielG]

Also some tests covering at least the check, type, modules, find and doc commands would be nice.