Tokenize dummy source map on words and new lines.

@Toilal, in #96, brought up a good point that improves line numbers
mapping. This takes that into consideration by improving the regex to
create newlines as separate tokens. It also makes tokens from white
space, letting non word characters be tokenized if they are surrounded
by whitespace.
    tokens.forEach(function(token) {
      if (token === '\n') {
        lineIndex++;
        charIndex = 0;
        node.add(new SourceNode(lineIndex, charIndex, name, token));
      } else {
        node.add(new SourceNode(lineIndex, charIndex, name, token));
        charIndex += token.length;
      }
    });

This will avoid bug when src ends with `\n`