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.
Unlike Markdown, which has 6 levels of headings, Gemini spec (p. 5.5.1)
only allows up to 3 shebang characters before a space. This adds an extra
space before levels 4-6 in order to comply with the spec.
Fixes#1.
This refactoring simplifies some originally quirky parts of rendering
code while also adding support for rendering emphasized text (handled by
the same routine the ordinary text already is).
