Add support for optional route parameters

[shiftyp]

Backbone has a syntax for optional parts of a route, for example: foo(/:bar) will match foo and foo/baz. Right now the Giraffe.Router#_reverseHash function doesn't support these parameters.

[ryanatkn]

Sorry for not documenting this - we were aware of the limitation when writing the function but didn't have the time to do it correctly. A PR would be welcome; I'll try to find the time to look into it after the holidays.

[bobthecow]

👍

[ryanatkn]

This is a partial fix. Optional params, like '/foo(/:bar)', should now work, but optional static paths, like '/foo(/bar)', remain broken for Router#isCaused. Fixing it looks like it requires some significant refactoring - a PR would be welcome if you find yourself needing it. Router#cause can't be fixed without manually specifying whether or not to include each optional part, which doesn't seem feasible.