Option to run tools in precompile / validation-only mode

[jack-pappas]

It would be a useful feature if fsharplex and fsharpyacc supported an option not to invoke any backend at all; in this mode, the lexer / parser specification would be compiled as usual, but the code-generation phase at the end would be skipped.

This would be used when running in an IDE to provide warning/error messages to the user. In the current version of fsharplex, the code-generation phase for the fslex-compatible backend accounts for over half of the tool's running time; skipping that phase would make it possible to provide almost-instant feedback to the user.