adds mask option

This allows to define a mask, that is used to mask the config before output. It uses https://github.com/wankdanker/node-object-mask for masking.
The `allow` parameter from node-object-mask is exposed as `maskAllowMode` option.

This is useful if you want to have one big shared config, but need granular control over what will end up where. You maybe don't need everything in your sass files, but something else in js...