Redesigned the customization variables a little.

I have replaced simple customization variables for Mallard commands with
composite types in order to separate the commands from their arguments.
This may look like an unnecessary complication right now, but trust me,
it will be worth it.
Minor adjustments for the sake of readability.
Added a function to check whether the command exists.

I have implemented the following two new functions:

  * The mallard-executable-exists-p function is a wrapper around the
    built-in executable-find function. It checks whether the supplied
    executable is present in the system, and returns t if it is.

  * The mallard-verbose-executable-exists-p function uses the
    aforementioned function and displays an error when the executable is
    not found.

I have also updated the mallard-run-command-on-buffer function to take
advantage of this newly implemented functionality.
Improved the documentation string for the mode definition..
Updated the documentation.
Changed the license (again).

After reading it again, I realized that CC-BY-SA is not really
appropriate for software and that it would be better to relicense this
package to make it easier to get it into distributions.
Updated version information.