Add `withStartIndices` option to add `startIndex` properties to nodes.

Fixes #7.
Refs https://github.com/fb55/htmlparser2/pull/106
When doing streaming parsing, the startIndex seems to get reset on each chunk.
Rather than try to hack the parser, I opted to simply disable the streaming test in this case, and document that startIndex+streaming doesn't work. My use-case doesn't involve streaming.