Feature: Config class for getting installation config without running the installation process

[vonNiklasson]

Adds a separate class called InstallConfiguration which holds the actual configuration for auto completion installation. The reason for this refactoring and feature was to add a way to get the path to the shell config without actually having to run to installation process.

Backwards compatibility is preserved while adding the possibility to install with the configuration class as well through the new function install_from_config(install_config: InstallConfiguration) -> Tuple[str, str].

Also added documentation info Return and Raises for the install function.

Example usage with the new class:

import click_completion.core

# Creates an InstallConfiguration class. This takes all parameters that the `install` function takes.
install_config = click_completion.core.InstallConfiguration()
# Run the actual installation process from the InstallConfiguration class
click_completion.core.install_from_config(install_config)

[vonNiklasson]

I am fully aware that this overrides the previous configuration setting on the lines above, but since this was the behaviour of the old way I don't want to break it with this PR.

[Konubinix]

LGTM. I would have put the whole logic into the class, even the install method. And then I would have called that an installer. But in its current state it looks ok anyway.

@glehmann : van you see a reason not to merge this upstream?