This adds support for fish auto-completion.

While there is already the click-completion project in click-contrib that provides fish support, it seems to me that:

1. Since there is already completion support for zsh and bash in click and adding fish is trivial it would seems logical to have it in the same place rather than require an additional dependency

2. Having it in click means that it is less likely to be prone to incompatibilities with other click extensions as it will not rely on monkey patching (currently I encounter issues using both click-completion and click-default-group at the same time).

See also issue #1060