More tools for manipulating urls

[hadley]

Fixes #425

[jonthegeek]

How would you feel about url_modify_path() (or maybe url_modify_path_append()) along these same lines? I know you can do it with the existing tools, but something equivalent to req_url_path_append() but for urls themselves is handy to (for example) avoid duplicate /'s in the same sorts of situations where you'd use these other url modify tools.

[hadley]

@jonthegeek hmmmm I think I'd prefer url_modify_relative() now that we have more relative path handling tools. That feels a bit more general to me.

[jonthegeek]

@jonthegeek hmmmm I think I'd prefer url_modify_relative() now that we have more relative path handling tools. That feels a bit more general to me.

That makes sense!