Add checkboxesField{,List} fields (based on multiSelectField{,List})

squashed:
    - Attrs are applied to checkboxes in checkBoxField
    - Fix checkboxesField{,List} attrs are applied to input instead of span
    - Fix checkboxesField{,List} attribute checked
Add missing exports for checkboxesField{,List}
Simplify in checkboxesField{,List}: no intermediate selOpts list
Merge pull request #603 from axel-angel/checkboxesField

Add checkboxesField{,List} fields (based on multiSelectField{,List})
cereal 0.4 fpco/stackage#121