allow checking for path non-existance

right now, using click.Path() allows a user to set the following params:
exists=True: aborts if path does not exists
dir_okay=False: aborts if path (exists and) is a dir
file_okay=False: aborts if path (exists and) is a file
(notice that for the each case, the opposite value will make the check to be simply skipped)

However, I believe that in many times, the programmer will want to write a program that verifies that a given path does not exists, (e.g. for purpose of creating it without overwriting anything)

for this case, I believe this set of parameters is better:
exists_okay=False: aborts if path exists
not_exists_okay=False: aborts if path does not exists
dir_okay=False: aborts if path (exists and) is a dir
file_okay=False: aborts if path (exists and) is a file

notice how before, users could think that exist=False will force the path to be non-existing, while now each parameter behavior is clear from it's name.

from a quick look at the code, making this change seems to be trivial.

Start from the click.Path entry point and inspect how its existing exists, dir_okay, and file_okay checks are implemented and tested. Determine how a non-existence check should interact with those options, then add coverage showing the requested paths are accepted or rejected as intended.