Explicit prefix and suffix modifiers

[blakeembrey]

Demo using explicit {} for prefix/suffix instead of implicit / and ., and nothing in other positions. This would allow for much clearer error and edge case handling of things like -:path* which is meaningless today, while /:path* is actually useful. It more closely aligns behavior with some other path matching libraries, such as Rails which uses () to denotes optional.

However, it does break compatibility as a result of fixing these quirks. As part of this PR there is a good attempt at ensuring the upgrade path is pretty easy, and it gives clear errors with a link and the exact part of the string where the error occurred. Mostly it'll be /:path? to {/:path}?. I've also disallowed /:path* which could be confused as * is now a wildcard, and writing that path is probably a bug. As a result, this does give a simple library upgrade path that adds back /:path?, /:path+, /:path* if needed by users.

It's also fairly easy to write a path rewrite script that'll update the syntax if people want it, both for Express.js 4.x and the current major release.