Added support for tracker links in paths, and filtering responses by header

[Cgboal]

Added support for using {tracker} in the paths file. When a tracker link is specified, meg will replace the {tracker} placeholder with the supplied link, along with some request metadata prepended to the supplied link in order to attribute callbacks. This is useful when testing for things such as SSRF, and can be used with burp collaborator.

I also included a commit which lets you filter responses by headers, although i see someone has also submitted a PR to filter responses by regex so this may be redundant.