FIX the react warning about input type checked

[bunyevacz]

react warns and removes the checked attribute from
input type=checkbox when markdown contains task lists.
So this plugin replace checked to defaultChecked=true
Note, defaultChecked do not work with react, =true is also
needed (react 16.6.3)

Example, before:

< input type="checkbox" disabled style="margin: 0px 0.35em 0.25em -1.6em; vertical-align: middle;" checked> first line

after:

< input type="checkbox" disabled style="margin: 0px 0.35em 0.25em -1.6em; vertical-align: middle;" defaultChecked=true> first line

React warning:
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.