Add 'chomp slash' feature

This is getting into a bit of an edge case territory, so it may not be best solved here.. but I was using one of the SecLists dictionaries and it prefixed the paths with /, which resulted in a // being generated by gobuster, which resulted in redirect behaviour on the website I was testing.

There is currently functionality to append a forward slash, I would see this as being the anti-case for that option.

-f	Append a forward-slash to each directory request (dir mode only)

It could probably be made more generic by allowing to append/chomp an arbitrary string (that defaults to /)

This may be easier (or at least nicer) to integrate into the CLI with #59

I would then see it being something similar to:

--append-slash
--append
--append /

--chomp-slash
--chomp
--chomp /

Edit: Re-reading this.. the current functionality is 'append', whereas I was thinking of 'prepend'. Maybe the current functionality of adding a slash could be controlled with a --prefix or --prepend and the current -f could be controlled with a --suffix or --append type flag.