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.
Using `&str` is likely better than `&String` because it's applicable in more situations and `&String` is basically the same thing (but more limited) than `&str`.