Emit long multi-line strings more nicely

YCP allows multi-line string literals. This means that long strings
often get written relatively nicely like this:

  s = "
  This
  is
  a
  long
  string.
  ";

Information about original string literal formatting is unfortunately
lost during parsing. All we know is that there is a long string with
newlines inside.

This commit tries to find such strings heuristically and emit them in a
bit nicer way than we did so far. It splits them to parts (one per line)
and uses Ruby |+| operator to concatenate them.

I also considered other alternatives, but rejected them for one reason
or another:

  * multiline string literals -- rejected because they would interfere
    with indenting mechanism in the emitter

  * heredocs -- rejected because they would interfere with indenting
    mechanism in the emitter

  * string literal combination using whitespace -- rejected because it's
    a relatively obscure Ruby feature and it doesn't work across lines

  * using |[...].join("\n")| -- rejected because other team members felt
    this is worse than using |+|

It's a pity that Ruby doesn't have CoffeeScript's |"""|. It would be
ideal to use here.