Support newly documented (but previously available) -M option

[nwwells]

See https://github.com/sphinx-doc/sphinx/pull/3938/files for the documentation, and https://github.com/sphinx-doc/sphinx/blob/master/sphinx/make_mode.py for implementation

Basically -M is a alias for -b with some additional handling for other features like a help and LaTeX.

Assuming this is just something we can pass through, it would allow me to do something like

make html --auto

and simply use sphinx-autobuild as the SPHINXBUILD option.

[pradyunsg]

Looks like a reasonable request!