Add support for keyword lists in tags

[hauleth]

Closes #23

[lexmag]

I'm still not sure if this is something that should be included for the following reasons:

1. StatsD protocol does not have key-value tags concept, perhaps it would be better to keep Statix aligned with the protocol
2. not everything implements String.Chars and ultimately only user knows how to cast their data to string

[hauleth]

1. StatsD do not support tags at all, not even comments (which would provide compatibility layer) so I do not think this is a problem.
2. I wouldn't say this is a problem either. However if this is a problem I can add limits to support only strings and atoms in lists/maps.