Fix ghCodeBlocks not being correctly parsed inside lists. Also, as a side
effect, fixes issues with consecutive lists and extra paragraphs being
added into lists.
Closes#142, Closes#183, Closes#184
Github Flavored Markdown supports tasklist by `[x]` or `[ ]` after list item marker.
This commit adds this feature to showdown through an option called "tasklists".
Related to #164
Consecutive lists we're previously being condensed into one unique list, with odd paragraph output.
This fix correctly splits lists, but does not change the weird paragraph output
closes#142