Clicking checkboxes

Hi,

If I use source like - [ ] checkbox and click on the checkbox, this warning is displayed:

Warning: Failed prop type: You provided a `checked` prop to a form field without an
`onChange` handler. This will render a read-only field. If the field should be mutable
use `defaultChecked`. Otherwise, set either `onChange` or `readOnly`.
    in input (created by ListItem)

It would be interesting to provide some way to actually handle the state changes other than changing the markdown source itself, but it sounds like a lot of work to handle the fact that the structure of the checkbox inputs is completely dynamic depending on the source. So it might be a good idea to make checkbox inputs readOnly by default to at least suppress the warning and make the behavior clear.

[rexxars]

Thanks chief! Will push out a new release in a bit

[philraj]

Here you go: https://github.com/rexxars/react-markdown/pull/121