Support static prefixes & suffixes in dynamic path segments

[FugiTech]

This address #707 by adding support for routes of the form prefix$var$suffix. This is technically a breaking change in that you can no longer have $ in the name of a dynamic path segment param. I'm unsure if that is something people do. If it is concerning I can adjust this to only support prefixes which would not be a breaking change.

I'm not familiar with the code base so I apologize if this is incomplete or broken. It seemed like there wouldn't be any changes needed to react-router as all the logic was in router-core. I wasn't sure if the cli needed changes.

[tannerlinsley]

I fixed up some more of the conflicts. It looks like a few examples broke with this (infinite loops?). So we'll need to hold off on this.

[tannerlinsley]

This would need to be updated with all of the new stuff, including file based route generation.