Link PlusArg to emulator command line options

- adds a mutable singleton (PlusArgArtefacts) to store information
  about Rocket PlusArgs
- adds methods to PlusArgArtefacts to emit C snippets that are
  consumed by emulator.cc for correct argument parsing and help text
  generation
- emits snippets in $(CONFIG).plusArgs via BaseCoreplex-set
  ElaborationArtefacts
- modify emulator/Makefrag-verilator to include $(CONFIG).plusArgs
- cleanup help text (docstring) for existing PlusArgs

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@gmail.com>