Add `--patch` flag to apply a Ruby patch from stdin

If `-p|--patch` flag was set while invoking `ruby-build` or
`rbenv install`, ruby-build will use `patch -p0 -i -` to apply a patch
from stdin to Ruby, Rubinius, or JRuby source code before running the
rest of `build_package_*` commands.

References #443
Shorten `rbenv install` usage synopsis

It was getting out of hand, so just use the single-character flags which
are documented below anyway.
Clarify which `rbenv install` flags get forwarded to `ruby-build`
Add examples how to use the `--patch` option to the Readme

[ci skip]
Merge branch 'apply-patch'

Closes #469