Create a simple template crate to support URL template hooks.

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.