Support multiple track locations and identifiers

[alastair]

How to expose these? A list? How to remove particular items?

[a-out-10-2]

The XSPF spec says this about track locations: "xspf:track elements MAY contain zero or more location elements...". This implementation only has support for one location string (https://github.com/alastair/xspf/blob/master/xspf.py#L229).

I figure these could be some options:

• Change location from a string, to an string[]. Add a method to append locations to list. This will likely break any code expecting a sole location string.
• Instead, keep the location string and add a field for additional_locations as an string[]. Maybe a get_locations() method can be called to compile the locations into one list. This will help maintain compatibility with other code, though may divert from the design of the spec.
• Make the location string, a list of locations delimited by some character (',', ';', etc). I'm not so sure about this one.