#6 Add stop zoom level field to cfcc_ref

[Bert Put]

cfcc_reference currently contains a zoom_level field
which specifies which zoom level a feature will appear at.
Change the existing name to start_zoom_level.
Add a new field: stop_zoom_level.
This would be used primarily at the low zoom levels
(i.e. the entire 48 states) to prevent the highly
detailed interstate segments from being displayed, and
have a low-detail series of segments display instead
(i.e. one line to describe I-10 from Pensacola to
Tallahassee)
This would have the effect of "swapping out" the
low-detail lines for the high-detail lines as the zoom
level increases.