Process ref field for consistency

[PetersonGIS]

The osm_transport_lines lines table has a ref field used for cartographic highway shield text. It sometimes contains non-conforming characters (e.g., ";"), multiple refs, or extra words (e.g., "Toll"). The data should ideally be cleaned up prior to its display. See the following examples:

This is how we'd like the shield to look:

This is how the shields can look bad if the data is not cleaned up:

An additional item to consider would be separating the number(s) so that the map maker can also choose to create number-only shields such as:

[PetersonGIS]

This was resolved in https://github.com/go-spatial/tegola-osm/pull/45