Add optional Gotham support

[kellenff]

This pull request implements the Gotham web framework's IntoResponse trait from #175, allowing items implementing askama::Template to be returned directly from Gotham handlers, like so:

#[derive(Template)]
#[template(path = "hello.html")]
struct HelloTemplate<'a> {
    name: &'a str,
}

fn hello(state: State) -> (State, HelloTemplate<'static>) {
    (state, HelloTemplate { name: "world" })
}

fn main() {
    gotham::start("127.0.0.1:7878", || Ok(hello));
}

[zzau13]

@djc In Rust 2018 we should not eliminate these extern crate?

[kellenff]

I've removed them in the latest commit

[zzau13]

Can you compact it to a single commit?

It is a very good work. Thank you.

[kellenff]

I had an issue squashing the commits, and had to recreate the branch. I'll create another pull request