Add ability to reset into safe mode from the REPL

[tannewt]

If no file system is writeable from the host computer do it automatically. If it is writable then require force=True kwarg.

[tannewt]

This is done in #474. You can:

import microcontroller
microcontroller.on_next_reset(microcontroller.RunMode.SAFE_MODE)
microcontroller.reset()