Documentation and validation for optional arguments for annotate/classify

[yetinam]

This PR cleans up the optional arguments for annotate/classify and their documentation. It contains three contributions:

• A Sphinx extension to automatically document all optional arguments.
• A parameter validation for the optional arguments. Any unknown argument passed to annotate/classify will raise a warning.
• Default values for all parameters are now defined centrally in the _annotate_args dictionary instead as scattered throughout the code.

Closes #100

[yetinam]

CI failures seem to be related to the scipy version and are most likely unrelated to the changes. I'll have a look into it in the next days. Please do the review anyhow.

[yetinam]

CI failure should be fixed now.

[jawooll]

Looks good, love the directive you've added for extensibility. Merging now.