Tighten definition of multiline strings (#16)

so that a `|||` in the middle of a multiline string does not end the string
according to the syntax highlighter unless it actually ends it
according to the language.

From the jsonnet spec:

> Text block, beginning with `|||`, followed by optional
  whitespace and a new-line. The next non-blank line must be prefixed
  with some non-zero length whitespace *W*. The block ends at the first
  subsequent line that does not begin with *W*, and it is an error if this
  line does not contain some optional whitespace followed by `|||`.

Prior to this change, the syntax highlighter looked for any sequence `|||`
as the end-of-string marker. This causes problems in multiline strings that
contain this sequence (this mostly comes up when providing example usage).

Example:

```
{
  help: |||
    To use this foo, you should specify a description like so:
      {
        description: |||
          Your description here.
        |||,
      }
  |||,
  ...
}
```

In this example, the line `Your description here.` would be considered outside
any string, with the part before it and the part after it being considered
two distinct strings.

We make the syntax highlighter use the same rules as the spec by making the
start pattern match `|||` followed by any amount of whitespace, then any number
of newlines (blank lines), then an amount of whitespace (*W* in the spec).

It uses the `\z( \)` construct to make this a subgroup accessible to the end pattern.

The end pattern uses lookaround to say that the start of the line must not match
the *W* from the start token, while still being any amount of whitespace then `|||`.

syntax/jsonnet.vim

@@ -110,7 +110,7 @@ syn match Type "\$"
 
 syn region String start='L\="' skip='\\\\\|\\"' end='"'
 syn region String start='L\=\'' skip='\\\\\|\\\'' end='\''
-syn region String start='|||' end='|||'
+syn region String start='|||\s*\n\+\z(\s*\)' end='^\z1\@!\s*|||'
 
 " Highlight python style string formatting.
 syn match Special "%\%(([^)]\+)\)\=[-#0 +]*\d*\%(\.\d\+\)\=[hlL]\=[diouxXeEfFgGcrs%]" contained containedin=String