Simplified the mallard-buffer-saved-p function.

I have split the mallard-buffer-saved-p function in two:

  * the mallard-buffer-saved-p function now only verifies whether the
    selected buffer is saved or not;

  * the mallard-interactive-buffer-saved-p function uses the aforementioned
    function to check whether the selected buffer is saved, and if it is
    not, it interactively prompts the user to do so.

In addition, I have also slightly simplified the function named
mallard-run-command-on-buffer.
Simplified command invocation.

I have enhanced the mallard-run-command-on-buffer to compose the command
on its own. Actually, this is how it was supposed to work all along.
Updated version information.