Markdown table formatter

Pads every cell so the pipes line up in the source, which makes a markdown table readable before it is rendered. It also builds a table from pasted CSV or tab-separated data.

Alignment changes nothing about how the table renders — markdown does not care whether the pipes line up — and everything about whether the source is readable in a pull request. A table maintained by hand drifts out of alignment within a few edits, and an unaligned table is where a missing cell hides. The separator row is the part that carries meaning. `---` is a plain column, `:---` left-aligns, `---:` right-aligns and `:---:` centres, and those colons survive here rather than being padded away. A table with the wrong number of separator cells does not render as a table at all, which is the most common reason a pasted table appears as a row of pipes. When building from pasted data, a tab is detected automatically — which matters because copying a range out of a spreadsheet gives tab-separated text, not CSV, and pasting it into a CSV tool produces one very wide column.