Add support for new click's version autocompletion flag

[oridistor]

As we all want arguements and options to be auto completed even if they are not the flag itself - new click versions have added a new flag to option called autocompletion which receives a function that returns a list of possible completions.

This can be seen here:
https://github.com/pallets/click/blob/master/docs/bashcomplete.rst

It can be nice to add a support for this feature as it'll allow us to have a non trivial auto completion quite quickly

[oridistor]

As I don't like to open an issue and not solve it, here is a CR that does the magic:

#69