Rewrite README with markdown

Updated it to mention that this engine is meant to do Steam
things too.
Have configure check for SDL2
Merge branch 'master' of https://github.com/sekaiproject/ponscripter-fork
Allow fullscreen and mute keys during autoplay.
Fix steam makefile and configure

Makefile was looking for header in the wrong directory which caused a
fresh compile every time.

Configure script didn't include the steamlib directory which failed if
no other libs were bundled.
Remove unneeded fix-timestamps line
Merge branch 'SteamApi' into SteamAchieves
Merge branch 'master' into SteamApi
Merge branch 'SteamApi' into SteamAchieves