Add the ability to enter a csv list of items to drush_choice_multiple

I needed this, and it seemed generically useful enough to have in Drush core (there is a suggestion for it at #582598: Submodule names in module package need guessing). The doxygen is below.

Let me know if you have any thoughts - the main things I am not sure about are if the $final_options concept is useful (or if we should limit it to a single built in "Done" option), as well as if we should try and cover eventualities by adding a $callback parameter (this could allow callers to alter the $selections, and perhaps even $options mid-stream in the process. I am thinking this could be handy where there are dependencies of some kind implicit in the options). If I don't hear anything in the next 2 or 3 days I am planning on committing as is.

/**
 * Ask the user to select multiple items from a list.
 * This is a wrapper around drush_choice, that repeats the selection process,
 * allowing users to toggle a number of items in a list. The number of values
 * that can be constrained by both min and max: the user will only be allowed
 * finalize selection once the minimum number has been selected, and the oldest
 * selected value will "drop off" the list, if they exceed the maximum number.
 *
 * @param $options
 *   Same as drush_choice() (see above).
 * @param $defaults
 *   This can take 3 forms:
 *   - FALSE: (Default) All options are unselected by default.
 *   - TRUE: All options are selected by default.
 *   - Array of $options keys to be selected by default.
 * @param $prompt
 *   Same as drush_choice() (see above).
 * @param $label
 *   Same as drush_choice() (see above).
 * @param $mark
 *   Controls how selected values are marked.  Defaults to '!value (selected)'.
 * @param $min
 *   Constraint on minimum number of selections. Defaults to zero. When fewer
 *   options than this are selected, no final options will be available.
 * @param $max
 *   Constraint on minimum number of selections. Defaults to NULL (unlimited).
 *   If the a new selection causes this value to be exceeded, the oldest
 *   previously selected value is automatically unselected.
 * @param $final_options
 *   An array of additional options in the same format as $options.
 *   When the minimum number of selections is met, this array is merged into the
 *   array of options. If the user selects one of these values and the
 *   selection process will complete (the key for the final option is included
 *   in the return value). If this is an empty array (default), then a built in
 *   final option of "Done" will be added to the available options (in this case
 *   no additional keys are added to the return value).
 */