Added ‘toText’ (tt) formatter for Showable instances

Solves a very common issue of formatting values that can be shown:

```haskell
ghci> import Text.Printf
ghci> newtype Issue = Issue Int deriving Show
ghci> printf "%s is valid: %s\n" (show (Issue 17)) (show True)
Issue 17 is valid: True
```
Using this you can write:

```haskell
ghci> fprint (toText % " is valid: " % toText % "\n") (Issue 17) True
Issue 17 is valid: True
ghci> fprint (tt % " is valid: " % tt % "\n") (Issue 17) True
Issue 17 is valid: True
```

I chose the name `toText` since `show` is obviously taken, I thought about using `repr` for the long formatter as Python does but `r` is already taken as a short formatter. Alternatives welcome. It is included in [text-format](http://hackage.haskell.org/package/text-format-0.3.1.1/docs/Data-Text-Format.html#t:Shown) which chooses the name `shown` for the formatter which is also valid but a short form is not as obvious.