Allow loading multiple and default databases

[robo9k]

magic_load() allows to pass in NULL to load the default database or passing in a colon separated (e.g. /usr/share/misc/magic:~/magic/custom-db) list of database filenames.
The Rust API should allow those use cases.

One possibility would be using a Vec<Path> for the load() function where an empty vec means "load default database" and "load these databases" in the non-empty cases.
In that case the load_default() function should be removed.