Added support for Attoparsec parser

Hi,
I've been using your Ini package, for me its the one that strikes the better balance between simplicity and number of packages it depends on. Thanks!

However, I need a function that would parse ini values using attoparsec, which offers a much bigger set of parsers than Data.Text.Read, and since attoparsec is already a dependency, this comes 'for free'.

To avoid breaking API changes, I've added a new function ```readValueAT```.

What do you think?
Thanks