Add support for request URL randomization / shuffling

[preslavrachev]

As requested in #35, the idea is to use regular expressions in various places, which will lead to different URLs being used when requests are spawned.

This current implementation only works with RegEx passed directly as part of the URL path or its parameters (wrapped within {{...}} ):

hey https://google.com/"{{[a-z]{10}}}"

Depending on your shell, the wrapping parentheses might or might not be needed.

Although this part can be merged, this is an ongoing effort and further changes would require the introduction of a local Request type first.

[iKarthik]

Any idea why this feature is pending merge? I can help if needed