Add FileID subproject

Implement small example of using libmagic to determine and output
type of an input file.

Use case is for file reader factory, where we need to know file type
to create a concrete reader instance.
Add files to test ROOT file ident

- isroot.root : actual ROOT file
- notroot.root : empty file
- mightberoot : plain text containing 'root' as first 4 chars (magic
  number for ROOT file)?