Add docblocks to tags

I was working on a PHP code completion plugin for vim and to make tag files more useful i needed the docblock comments for the individual tags.
The body of the docblock comment is just copied in as-is (with replacing the `\n` `\r` `\t` with the string literals `"\n"` `"\r"` `"\t"` so they still on one line). These comments can be used to show as documentation for the completions, or parse them for return types for methods, or types of properties and parameters. To make this work I've added a new extension field named docblock and modified the php.c to capture comments that look like docblocks as it reads sees them and add them to the tag entries.
Without having these in the tag file the plugin would have to look up the tags in their source files and that's a big performance hit.

I'm not sure if you find this useful or not, but your rewritten PHP parser is tremendously useful for me, so i wanted to contribute. (The commit dcfdff74de0e94d90305f0cfa42f859f43bf7c2b is unrelated to the docblock feature, but I've found that useful too and can't exclude from this pull request, you can cherry-pick though).