Added support for raw HTML in tables. Cell values and text are
HTML-escaped by default; to emit raw HTML instead, mark whole body
columns with lt_html(), or wrap the text passed to
lt_header(), lt_label(),
lt_spanner(), lt_footnote(), or
lt_note() in I().
Added lt_export() to save an lt table to a file:
.html (an HTML table, optionally baked to a static
<table> via Node.js or a headless browser so it needs
no JavaScript to view), .pdf (a vector PDF), or
.png (a raster image). PDF and PNG are rendered in a
headless Chromium browser and cropped tightly to the table by
default.