Merging LinkedIn's header_rewrite improvements upstream

At LinkedIn we have following improvements to header_rewrite module:

• Variable expansion for add-header;
• `Counter` operation that increments counter on rule triggering;
• Misc:
  • Whitespace and trailing spaces cleanup;
  • Const correctness;
  • -Wextra fixes;
  • Improved docs.

We would like to push them upstream to reduce diff between our internal branch and origin. I'll rebase our changes on top of you master and submit them to review via github's pull requests[1].

If you have any complaints/suggestions - please feel free to express them here.

[1] https://github.com/apache/trafficserver/pulls