This adds support for Markdown table rendering with
github.com/olekukonko/tablewriter. Tables are rendered as ASCII text
in a preformatted text block. Cells are hard-aligned with spaces.
tablewriter options are not yet configurable, although they should be.
For now, extra formatting inside tables is omitted.
Fixes#2.
This also introduces a simple program, md2gmn, that is meant for testing
the renderer on Markdown files. Nonetheless, it can be used as
a standalone tool as well.
