Recursive directory creation for target

When copying a single file or nested directory, copy the asset recursively. For example:
    
    src: asset/media/css/styles.css
    dest: media/css/styles.css
    
would fail, because media/css directory doesn't exist within metalsmit's destination (or build) directory. Specifying a new `createDest: true` option, will create the required rectories recursively. `createDest` defaults to false, to preserve the original behaviour.