Added lxml.html.CheckboxGroup.value_options property

[alxistr]

In lxml.html.CheckboxGroup.doc exists description for property .value_options that is not implementated. Added method is the same as in RadioGroup.

[scoder]

This is lacking tests.

[alxistr]

This commit append 7 lines that is not influence for something. Moreover, this code should be there for a long time, since it is documented. I'll search similar tests for RadioGroup, and if they are exists I'll add them.

[scoder]

I'm inclined to think that the documentation was just a copy&paste oversight. And I do not see the relation between (the potential lack of) tests for RadioGroup and any tests for CheckgroupBox.

But I do agree that this would be helpful, so I'm asking you to finish up your changes (which obviously includes tests) so that I can merge them.

[alxistr]

In tests folder there is no any tests for forms and controls. Should I add this too? Even there is no html file with forms and controls, only one with simple divs (lxml/tests/shakespeare.html).

[alxistr]

I mean, If I add the tests for it commit that it will have to make too great additions like append html file with

etc. Because there is no other similar tests.

[scoder]

The tests for lxml.html are in

https://github.com/lxml/lxml/tree/master/src/lxml/html/tests/

Most of them use doctests.

[alxistr]

I think it is finished. Is it fine that commits have same names?

[scoder]

I prefer either having tests and feature in the same commit (if they are reasonably small) or giving each separate commit a telling name, e.g. "added tests for new .value_group property" or something like that for the commit with the tests. However, given that this would best have been one commit, I'm fine with having one commit comment for both.

Thanks for the contribution.