Support helperText in a BooleanInput

[kctrlv]

What you were expecting:
I would like to be able to set a helperText in this input. I can do this in other inputs, such as
<TextInput helperText='abc123' /> and
<RadioButtonGroupInput meta={ { helperText: 'abc123' } } />
but currently there is no way to set it for a BooleanInput.

Other information:
The reason is that FormHelperText is not included in the BooleanInput.js file. If it were, it would probably be as simple as copying lines 161-164 from RadioButtonGroupInput.js

[fzaninotto]

Hi, nice idea for an enhancement. Feel free to submit a PR about it.

[fzaninotto]

Fixed by #3339