Corrected Two Markdown Parsing Bugs

Corrected a couple of bugs related to Markdown parsing.

• The App Preference setting to select the desired Markdown parser was not working – Notenik’s parser was always being used, no matter what the setting. This has been corrected to once again allow use of Down or Ink instead of Markdown’s own parser.

• Hash characters (also called pound signs) were sometimes causing lines to be formatted as headings when they should not have been. A couple of changes were made to prevent this from happening. The first change was a straight-up bug fix. The second change was a matter of better heading identification. If a line starts with a single hash mark, and is immediately followed by a character representing a digit (with no intervening space), then the hash mark will be treated as ordinary text, rather than the beginning of a heading line.