Create a simple template crate to support URL template hooks.

[charlespierce]

Currently, the URL Template hooks introduced in #241 use a simple string replace to resolve a template. Going forward, we should make this into a separate crate and add some additional functionality, such as escaping the brace characters.

We likely don't need a fully-fledged templating engine, since we are only dealing with URLs, but having the logic in a separate crate will make it easier to iterate on the behavior.

[jayeshmann]

Is this available? Any PRs being worked on?

@charlespierce can you assign this to me I'm interested in to contribute