Offer a table in the slash menu

Nothing could make one by hand: markdown has no shortcut the editor
wires up, so a table arrived only from the agent or from a paste. Rails
that grow a table you cannot create are half a feature.

Row headers only. `includeHeaders: true` would mark the first cell of
every row as a header as well, which draws a bold shaded column that GFM
has no way to write — the table would claim something on screen that the
saved source does not say.