Implement syntax in gh #1185 for @import options.  Implement multiple & ...

...less.

First step in implementing syntax for @import options, proposed in
https://github.com/cloudhead/less.js/issues/1185#issuecomment-13710620
(steps (1) and (2)).

I've implemented the 'multiple' and 'less' options.  One could trivially
add 'once' and 'css' options as well, if there was need.  Proposed
"silent" and "inline" options are deferred for future work.

I left the existing "@import-multiple" and "@import-once" syntax in place,
although the proposal is for this to be deprecated once the new option
syntax is in place.