[bikeshed] Now with line-numbers in errors!

I finally realized how to do accurate, pervasive source-code line
numbers in Bikeshed error messages!

Some happen automatically; errors in your metadata or any datablocks
(<pre class=whatever> things that Bikeshed specially processes) get
line numbers all the time.

Most others you'll need to opt into: run `bikeshed spec -l` (or
`--line-numbers`) to get line-number output.  Unfortunately this mode
is a very hacky, regex-HTML-parsing sort of thing, so Bikeshed
automatically does a "dry run" when you opt in, and doesn't actually
save any output files.  But if this does anything, you have errors in
your document anyway, so that's probably fine. ^_^  When you've fixed
things, just run it without line-numbers and it'll save as normal.

Hopefully this'll help avoid any more annoying 10-minute bughunts!

~TJ