Optimize node classes imports

[zurk]

It helps to replace such huge imports

from lookout.style.format.feature_utils import (
    CLASS_INDEX, CLASSES, CLS_DOUBLE_QUOTE, CLS_NEWLINE, CLS_NOOP, CLS_SINGLE_QUOTE, CLS_SPACE,
    CLS_SPACE_DEC, CLS_SPACE_INC, CLS_TAB, CLS_TAB_DEC, CLS_TAB_INC)

with just

from lookout.style.format.feature_utils import CLASS

(naming should be better.)

Also, it is a good idea to add such function as is_emply, is_indentation etc.

I can take this issue but after the release.