Adding definition for selecting an input by the group's label.

Allows for a step like: `I select "Admins" from "Groups" input group`

Adding it to BasicContext as it seems useful to be used in
other places, not just the CMS admin interface. It could be used
frontend forms that use CheckboxSetField, or OptionsetField,
for example.

Thanks to @srizzling for writing the definition for this step!
Merge pull request #15 from halkyon/select_radio

Adding definition for selecting an input by the group's label.