chore: makes the dependency `docopt/docopt` optional

[nunomaduro]

This pull request makes the dependency docopt/docopt optional - as most of the libraries that depend of this project don't use the command line tool.

If you agree with this, I can also update the docs accordingly.

[marcioAlmada]

@nunomaduro is there any particular issue/incompatibility happening with docopt as a dependency?

[nunomaduro]

Yes - It's really about having dependencies that are not needed in order to yay to work. Using the CLI tool is not really the use case of this library.

In my opinion, within the CLI tool, we could check class Docopt exists, and if do not exists we throw and exception say: The package Docopt is required for the CLI mode: please run composer require docopt/docopt. Or similar.

I am whiling to work on this if you want.

[marcioAlmada]

Agreed:

• [ ] add a check for Docopt and a error message on bin/yay
• [ ] update README.md before the first yaycommand line usage example

[nunomaduro]

No plans on working on this at the moment.