Support mixed dotnet langs projects (F# mostly)

@DustinCampbell Last I asked about possibilities for autocomplete of F# namespaces and types in omnisharp and the answer was it won't work because roslyn doesn't compile F#.

I think you may have mistaken my question, all I meant to ask was would metadata autocomplete work. (like it does for packages already).

This should be a small fix.
Currently the project loader tries to load the project with roslyn and if it fails ignores the project.
The fix should be to then try to find if there is already a built assembly and load that.

Like @enricosada did for FSAC https://github.com/fsharp/FsAutoComplete/pull/173