Change `parse-markdown` to accept either path? or string?

When caller supplies path?, a couple benefits:

- Because we call `file->string` we can supply `#:mode 'text`; caller
  doesn't need to remember to do so.

- We can set Parsack's `parse-source` parameter to include the
  filename in syntax error messages.

Also:

- Update README to explain this.

- Update suite-test.rkt to use this.