Allow to read ILD files from plain data

[gavrushkin]

Hello Brendan,

Here is a small improvement that allows to read ILD directly from memory without files on disk, it's necessary sometimes in LaserOS app (core part of the app is a GPL-library ldCore, liblzr is a part of it).

I'm not sure if name ilda_open is ok, please let me know if it's better to rename it, maybe ilda_open_buffer or something like this.

Also just FYI we use a small Cmake file to build required parts of liblzr on different platforms. You can adjust it to build your lzr library with cmake, cmake is nice :)

Thank you,
Sergey