Add support for complex tags using dashes, periods, and underscores. 

This PR adds support for more complex tags by allowing the use of dashes, periods, and underscores in the tag field.   With these, one could create a tag that describes a very specific type of log, e.g. "app-01.dfw_apache_access_prod" could tag logs coming from Apache's access log, running a server named app-01  in a production environment in the Dallas Fort-Worth datacenter.