Add multi line text interpolation support

[hishnash]

Currently Splash does not support multi line string interpolation for swift.

However this is a feature of swift and is very useful to use when blogging as keeping the line length short is important for mobile/iPad viewers.

Before:

After:

This fix now supports multi line text interpolation within multi line strings.

It however does not support dealing with the edge case were there is a string nested within the interpolation that itself includes a closing brace. I feel that is quite a bit of an edge case and supporting it would add a lot of complexity.

let str = """
Hello \(name ?? "closing-branch-within-string-)")
"""