Add non-failing getter for params.

Title says it all. Right now Index is the only way to get a value out of Params, which
will fail if the key is not present.

While I understand the motivation behind this - you should only be accessing params
that come from the url it limits certain useful patterns, like the ability to decode
params to use Decodable and convert Params to a struct.
Perhaps this should take `&str` and return `&'a str`?