Allow single element wildcards in the last path segment to end with an extension

Fixes https://github.com/robfig/revel/issues/333 and https://github.com/robfig/revel/issues/459

The implementation feels a bit ugly to me, since it's not clear that it's keeping the extension in the last wildcard element and then trimming it out later. @robfig you may already have a better way in mind.

Efficiency delta is negligible:

| Test        | master           | new  |
| :------------- |-------------| -----|
| BenchmarkTree100      | 809 ns/op | 813 ns/op |
| BenchmarkRouter      | 1789 ns/op | 1793 ns/op |
| BenchmarkLargeRouter      | 1340 ns/op      |   1336 ns/op |
| BenchmarkRouterFilter | 3138 ns/op      |    3143 ns/op |

(including the router benchmarks from the revel test suite)


cc @brendensoares as you're not currently watching this repo.