add opts processing to shocco

This converts setting custom title to getopts -t, and adds a -c
for a custom css url that can be passed when we build the docs. -h
is added for completeness.
fix help function definition to be posix shell compliant

I let the bashism slip into this patch, which breaks it when run under
ash on ubuntu (hence failing in the devstack doc job).

http://unix.stackexchange.com/questions/73750/difference-between-function-foo-and-foo
seems to indicate that this is the syntax that will work.