Add possibility to provide a custom magic-file

I would like to use `stream-mmmagic` in my project, but I have the problem that it does not detect `video/m2pt` files as the magic-number is not part of the default 

[This post](http://unix.stackexchange.com/questions/129081/file-doesn-t-show-proper-mime-type-for-m2ts-files) describes how to write a magic-file that detects m2ts-files, but `stream-mmmagic` does not offer a way to provide a custom magic file. This pull-request includes that ability. If you think you'd rather do it another way, please do so. I would however, very much like to use this module as it is exactly what I need.