Mpy cross sync

[BrianPugh]

• [new command] run runs a file.
• [new command] exec executes a python statement.
• sync improvements:
  • New argument dst; defaults to "/"; folder to sync file into.
  • New argument ignore; uses gitignore pattern matching to ignore files during sync.
  • New argument mpy_cross_binary; if provided, will compile .py files into .mpy prior to syncing.
  • Can now sync a single file.
  • Now keeps all untouched files if keep=True.
  • Misc bug fixes.