Add test for consecutive newlines in string node.

SourceNode should skip over all newlines in the node, but it
currently skips every other consecutive newline. This is due to RegExp
behaviour with the 'g' flag - the RegExp object will track its
position in successive calls to `test` (or `exec`) for the same string.

See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/exec#Example.3A_Finding_successive_matches
Remove `g` option from REGEX_NEWLINE.

This ensures that successive calls to `test` for newline characters
will always return `true`. Using `g` will cause every other `test`
call to return `false` and is not required for the other usage.
Merge pull request #116 from jgoz/consecutive-newline-bug

Fix consecutive newline bug in `SourceNode`