Support custom inputs

[tcbegley]

Bootstrap and reactstrap have custom input components, see for example the reactstrap documentation. These are inputs that don't use the native browser form elements like radio buttons, but instead render custom elements using CSS.

In our case, it would be most relevant to Checklist, RadioItems, Checkbox and RadioButton. The file upload and select dropdown we don't really support anyway.

In the case of the Check* components, we can also support the toggle style.

There's a few things I think to be decided:

• Do we make additional components for this, or add a prop custom to the existing components that can be used to choose which underlying component to use. If the latter, what should the default be?
• The CustomInput component in reactstrap needs id to be specified to work. Since this would have to be the case anyway when using the component in a callback it will mostly not be a problem, but it should fail gracefully if for some reason id is not set. If we decide to support CustomInput through existing components, we could fall back on native HTML elements if id is not set, which might be the best solution.

[tcbegley]

Resolved by #238