Make more tolerant to whitespace

[athas]

The remarks parser is very particular about where it does and does not expect whitespace. The TAs who write these corrections generally are not. I think the parser could be made more lenient. Two cases I can think of:

• The parser fails if a list item begins in the first column.
• The parser fails if the file does not terminate in a newline.
• The parser fails if there is trailing whitespace after the points assignment (e.g. 2/2 ).