(#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}'