multiline strings enhancements

[rogue-one]

following features support for multiline strings will go a long way.

• support for substitution for multi-line strings
• support for scala's stripMargin like feature.
  ie remove leading and trailing white-spaces without relatively changing the indentation between each line. This is required if we have any indentation sensitive content in the leaf string node of the config, say for eg a python script. below I have used triple quotes to distinguish this from normal multiline string.

   conf = {
      language = python
      code = '''
             |for item in range(10)
             |     <some expression>
             '''
    }

[havocp]

Substitution could use the backtick "template literal" syntax from JS: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals
This seems fairly straightforward if we have anyone willing to do the spec/code patch.

I don't really know how to do stripmargin. There are limits to what we can do without making HOCON into a programming language... but maybe there's a clever solution. There is a workaround now I believe (use multiple string literals and allow them to be concatenated) which doesn't seem terrible, so I don't know if we'd want to do anything complex here. But it could be worth exploring options and proposing how it would work.