(HI-127) New interpolation function: literal

This patch adds a new Hiera interpolation function: `literal`. This function
simply returns whatever was passed to it and halts recursion.

The primary use of this function is escape the Hiera escape character "%". For
example, the following data:

    %{literal('%')}{some::key}

Interpolates to:

    %{some::key}
Merge pull request #185 from Sharpie/HI-127-escaped-lookups

(HI-127) Add 'literal' function for escaping lookups