Allow escaped json to survive rendering

[jbohanon]

Add option to .dump() strings instead of .getstd::string().

this is useful in cases where the output of the rendered template is expected to be valid JSON. For example:
Input:

{
  "foo": "\"bar\""
}

Template:

{
  "FOO": "{{ foo }}"
}

Output (without allowing escaped strings):

{
  "FOO": ""bar""
}

Output (allowing escaped strings):

{
  "FOO": "\"bar\""
}