Titlecase trait

[carlocorradini]

This library is amazing, and it works beautifully.
Currently, the only way to convert a string to title case is using the given function:

let text = "hello world!";
assert_eq!(titlecase(text), "Hello World!");

Even if it works without any issues, it is not really "idiomatic" and is similar to certain JS/CPP/etc... code.
The following code should be our final goal, where the titlecase method is applied directly to the string using the Titlecase trait:

use titlecase::Titlecase;

let text = "hello world!".titlecase();
assert_eq!(text, "Hello World!");

What do you think?
I'll create PR 🙌