Stub read-only support for SubViewer

Since it uses the same file extension as MicroDVD (*.sub), add read/write
filtering capability to extension_to_format(). This is rather ad-hoc
solution, but may work for now (ie. to disambiguate *.sub saving w/o
specifying the format via keyword).

Consider changing MicroDVD keyword from "sub" to "microdvd" to be more in
line with the rest. This would probably warrant an alias to "sub" for
backward compatibility.

Review this after SSA/ASS is implemented, since that also uses aliases
for the same format class. It is necessary to tell the format class what
keyword we want it to handle. MicroDVD class could just ignore
the dichotomy.