Add seq-based overload for Docopt.Parse

[rmunn]

Description

Fix #2433 by adding an overload to Docopt.Parse that takes an IEnumerable<string>. All existing unit tests continue to pass after this PR.

TODO

Feel free to open the PR and ask for help

• New (API-)documentation for new features exist (Note: API-docs are enough, additional docs are in help/markdown):

Help needed: there's no API documentation for Docopt yet, and as I tried to write it, I realized I don't quite know what to write to explain the main Parse() implementation.

• unit or integration test exists (or short reasoning why it doesn't make sense):

Help needed: there are no unit tests for Docopt, and I don't feel up to writing them at the moment.

Note: Consider using the CreateProcess API which can be tested more easily, see https://github.com/fsharp/FAKE/pull/2131/files#diff-4fb4a77e110fbbe8210205dfe022389b for an example (the changes in the DotNet.Testing.NUnit module)

• boy scout rule: "leave the code behind in a better state than you found it" (fix warnings, obsolete members or code-style in the places you worked in):

This is a one-line PR, so I didn't really look at any other code except the Docopt class, and I didn't see anything in the Docopt class that needed fixing.

• (if new module) the module has been linked from the "Modules" menu, edit help/templates/template.cshtml, linking to the API-reference is fine.

N/A

• (if new module) the module is in the correct namespace

N/A

• (if new module) the module is added to Fake.sln (dotnet sln Fake.sln add src/app/Fake.*/Fake.*.fsproj)

N/A

• Fake 5 API guideline is honored

[matthid]

Help needed: there are no unit tests for Docopt, and I don't feel up to writing them at the moment.

Ironically I think that's the component with the most tests ;)

In any case, thanks a lot :)