Flex schemes requires many paths

[qfox]

Atm we have one .path method that can't return many paths for flex schemes.

For paths:

/level/b@desktop.css
/level/b/b@desktop.css
/level/@desktop/b.css

We have confusion because path can return just one string in result:

flexScheme.path(BemCell.create({layer: 'desktop', block: 'b', tech: 'css'}));