Render "compatible" with Options::Type::Boolean

This moves the responsibility for rendering the "compatible" option from
the VimrcRenderer to the Options::Type::Boolean, which was already
responsible for rendering the form fields.

This will reduce duplication but does couple the form field rendering
concern with the rendering of the actual option. This is acceptable
since it seems likely for now that future options would act the
same in both areas.
Make Type.new return an instance of a class

`Foo.new(...).new(...)` is ugly.

This commit offers an improvement by passing both arguments to
`Options::Type.new`.
Remove unused default value from options

This is no longer used because the default values are getting set from
`OptionSetForm#initialize`.
Make Capybara locator more specific
Test Option and Options::Option separately

This includes some duplication but should make it easier to add additional
tests for future option types.
Move expandtab option into configuration