[FEATURE REQUEST] adding support for add_slash + extensions

[cortantief]

Is your feature request related to a problem? Please describe.
Sometimes on pentesting purpose we found server that force the en slash to detect a directory.
For example /cgi-bin => not found while /cgi-bin/ is valid.
But we want at the same time to be able to give to ferox access to some extensions.

Describe the solution you'd like
I made an implementation here.
In that implementation I treat forward slash as a particular extension with a specific format.

Describe alternatives you've considered

Additional context
Sorry I didn't fully followed the procedure, I didn't read at first the MD file since is my first contribution on a project I just forked it.