Add negation matcher to hoverfly

[kapishmalik]

resolves => #1101

[tommysitu]

Thanks, I will test it, is this how you use it?

{
   "matcher": "negate", 
   "value": "something else"
}

[kapishmalik]

Hey @tommysitu yes, that is way we can test it. I have added integration tests as well for reference. Also, I will add documentation once approach looks fine.

[tommysitu]

It looks great 👍