(#127) Allow escaping %{...} in hiera data so that hiera does not try to perform a replacement

This change allows the escaping of %{...} with %%{...} so that the result is literally '%{...}'

It also support escaping the escape eg:
 var = 'value of var'

    %{var}      : 'value of var'
   %%{literal}  : '%{literal}'
  %%%{var}      : '%value of var'
 %%%%{literal}  : '%%{literal}'