The type of a config value depends on the tool that interprets
it. Parsing eagerly can lead to a situation where we return a bool
instead of a string or a number.

Let the user specify the type themselves by passing in a (str, type)
tuple into the mapping interface.

Passing a tuple to the mapping interface isn't the best of interfaces,
as the key is only the string.

Instead, expose `Config.get_bool()` and `Config.get_int()` methods to
parse the values as per the git-config rules before returning the
appropriate type to the user.

The mapping interface itself returns a string.