Support for geocoding of Interstate Exits

[robe]

Addresses such as

Rt 6 Exit 20

This requires pulling out the ramps from tiger data (MTFCC code S1630) that intersect with the matching highway. This will only be done if exit/Ext. appears in the address (MTFCC code S1100). We can probably also reduce our matching to just MTFCC S1100 roads if an Exit is requested. though how to get the exit number is a mystery to me.