smpeg2 2.0.0 (new formula)

Introduce a separate formula of `smpeg2` for the latest version (2.0.0) dependent on `sdl2`.

An update for the old `smpeg`, which temporarily moved out to `homebrew-headonly`, is also posted (Homebrew/homebrew-headonly#79). 0.x version should remain as is because many applications still assume smpeg with SDL 1.2.

Summary of difference:

1. Uses `sdl2` instead of `sdl`.
2. Only `smpeg2` has `head`.
3. No `gtk+` dependency. Looks like it's been dropped in the upstream.
4. `--with-plaympeg` added to make plaympeg building optional. Otherwise it could have conflicted with plaympeg from old smpeg.
5. chmod trick removed. It seems working fine without permission fix.