Aligner markup

An aligner is a special single-line token which specifies the alignment of the subsequent text. It is essentially 'modal' - all text from the token onward (until another aligner is encountered) is wrapped in a <tw-align> element (or unwrapped in the case of left-alignment, as that is the default).

the alignment: for instance, one = to the left and three =s to the right produces 25% left alignment.

Any amount of whitespace is permitted before or after each token, as long as it is on a single line.

Example usage:

==>
This is right-aligned
  =><=
This is centered
 <==>
This is justified
<==
This is left-aligned (undoes the above)
===><=
This has margins 3/4 left, 1/4 right
  =><=====
This has margins 1/6 left, 5/6 right.