Added checklist support using github markdown `[ ]` tasklist syntax

Todo elements are rendered as `[ ]` or `[x]`

A list of checklist elements is rendered as `* [ ]` instead of just `[ ]` elements to resemble more github syntax, and to not requite to leave spaces.

Without the extra parse:

    [ ] element1

    [ ] element2

    [ ] element3

With the extra parse:

    * [ ] element1
    * [ ] element2
    * [ ] element3