Add header matcher support

[esammer]

It should be possible to match the presence, absence, and value(s) of HTTP headers.

[rajkumarGosavi]

Hi,
I am interested to work on this issue

[esammer]

@rajkumarGosavi Happy to have the help. If you want to put up a PR that covers this, I'd be happy to merge. Thanks!

[rajkumarGosavi]

May I know what is the aim that is to be acheived?

[esammer]

Not sure entirely what you mean, but in terms of functionality, the intention was to match on any header value so the following works:

NewRule(
  &matcher.HeaderMatcher{
    Header: "Accept",
    Value:    "application/json",
  },
  RespondWithStatus(http.StatusOK)),
)

...or something like that. The other matchers are good examples of the pattern. It would be nice to also provide a convenience constructor at the top level package like the other matchers.

[rajkumarGosavi]

#7