simple-markdown: parse tables with and without pipes

Summary:
github flavored markdown supports two types of tables:

    | leading | pipes |

and

    no-pipe | tables

this supports those as `table` and `nptable`. The logic and
regexes are adapted from marked.js

Test Plan: `make test`

Reviewers: emily

Reviewed By: emily

Differential Revision: http://phabricator.khanacademy.org/D12698