Markdown.pl does not appear to escape \<[nonwhite] sequences.  Sigh.
Tweak the previous `Markdown does not escape...` commit to simply push
out the backslash and back up to the start of the <[nonwhite] sequence,
so -fnohtml will continue to work.
A \< at the end of the input is exactly the same as \<(space)
Clean up test cases for \<; need one for whitespace, one for EOF, one for text
Use styles for table cell alignment instead of "align="
Tweak isthisspace() to consider all characters <= ' ' as space
(including ^C and EOF)