Allow to set cache via 'neobundle#cache_file'

This also provides neobundle#commands#get_default_cache_file, which can
be used to use the same base for different cache files.

This is useful when you have multiple Vim profiles, e.g. "light" and
"default", which use a different set of bundles and therefore should
use a different cache, too.
Merge pull request #351 from blueyed/allow-to-override-cache_file

Allow to set cache via 'neobundle#cache_file'