Hi 127 escaped lookups

The first commit introduces a new Hiera interpolation function, `literal`, which simply returns its argument and breaks out of recursion. This new function can be used to escape Hiera interpolation by doing:

    %{literal('%')}{some::key}

Which resolves to:

    %{some::key}

Since that syntax is quite verbose, the second patch adds an alias that can be used as a shortcut:

    %{%}{some::key}

Resolves to:

    %{some::key}

This should address [HI-127](https://tickets.puppetlabs.com/browse/HI-127)