Mention and_then combinator

As for https://github.com/bodil/bodil.github.com/blob/master/content/parser-combinators/index.md

It would be nice to have include
```rust
fn and_then<'a, P, F, A, B, NextP>(parser: P, f: F) -> impl Parser<'a, B>
where
P: Parser<'a, A>,
NextP: Parser<'a, B>,
F: Fn(A) -> NextP,
{
move |input| parser.parse(input)
.and_then(|(next_input, result)| f(result).parse(next_input))
}
```
as an alternative to
```rust
fn and_then<'a, P, F, A, B, NextP>(parser: P, f: F) -> impl Parser<'a, B>
where
P: Parser<'a, A>,
NextP: Parser<'a, B>,
F: Fn(A) -> NextP,
{
move |input| match parser.parse(input) {
Ok((next_input, result)) => f(result).parse(next_input),
Err(err) => Err(err),
}
}
```
Maybe it might be good to mention the previous version as well.