Handle click.exceptions.Exit, introduced in Click 7.0

[theazureshadow]

First we attempt import of click.exceptions.Exit, but if it doesn't exist we polyfill with a local exception class ClickExit (that will never be thrown, of course).

Test approach:

• Manually tested using the example code in click v7 breaks click_repl #50.
• Tested against python 2.7.15 and 3.7.0
• Tested against Click 6.7 and 7.0.

Fixes #47
Fixes #50

[untitaker]

Done, thanks!

cc @WhyNotHugo