Add support for different levels namings & schemes

[shkarupa-alex]

bem-deps uses bem-walk inside gather function https://github.com/bem-sdk/bem-deps/blob/master/lib/gather.js

But now there is no way to provide levelsMap to bem-walk.

const walker = walk(config.levels, { defaults: config.options });

This line should look like

const walker = walk(config.levels, { levels: config.options.levels, defaults: config.options });

[qfox]

Nice catch! Would be great if you provide a PR ;-)