Added `format()`

While the StringBuilder has sprintf format helpers, the main Str class did not. Well now it does.

Str::s('There are %d monkeys in the %s')->format([5, 'tree']); // There are 5 monkeys in the tree

Removed one method and added another.

• replaceExact() was removed. I have no idea what I was thinking when I created it. Talk about over complicating things.
• is() has been added. Borrowed from Laravels str_is() method. Basically a poor mans wildcard regular expression.

Alright lets release this already.

I have been using this myself for ages now, so too have a few other people by the looks of the packigist stats, 2k worth of downloads, I'm flattered :)

Ideally I would have liked to polish off the documentation further but that will come with time.