Add app-emacs/rust-mode.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
Add app-{emacs,vim}/rust-mode/metadata.xml.

I don't have a mail address at gentoo.org, so put my gmail address for
now.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>