Add support for raw tags

[magiconair]

Raw tags add arbitrary route commands to the routing table. The current approach needs review but works.

By prefixing a tag with fabio the following text will be added to the routing table.

See #87 Fixes #455

Note

Before we merge this the service method in https://github.com/fabiolb/fabio/blob/master/registry/consul/service.go#L68-L163 needs to be refactored to do only text transformations and there need to be unit tests. Also, the raw tag prefix needs to be configurable.