Add a PREFIX variable, add opam file

[KaiSforza]

No description provided.

[KaiSforza]

Note that I have no idea how opam works, I'm just guessing and reading some manuals, so if this is absolutely awful, feel free to tell me.

[hugomg]

Since Windows is an important platform for this program, I took some time to think about how to best support both platforms before merging this just yet.

Opam is the clear favorite on Linux and appears to be the standard Ocaml package manager moving forward but its not ready yet for use on Windows (users need to install opam from source here and the executables it produces require cygwin.dll to run). Because of this I've decided to keep supporting WODI and to try to write a GODI package as well. This could possibly require me to tweak the makefile a bit so I'm gonna wait until I finish the GODI packaging before merging this.