futurize -a is not affected by fixer setting

[QuLogic]

futurize -1 -w -n -a . will add print_function, division, and absolute_import (PS, can those be changed to be in alphabetical order?).

futurize -f libfuturize.fixes.fix_absolute_import -w -n -a . will add the above, plus unicode_literals and future imports.

It would be preferable if, when explicitly specifying a fixer, that only the __future__ imports relevant to that fixer were applied, in much the same way selecting stage 1 (-1) only uses imports that are "safe". This allows one to create smaller commits that would be easier to review and hopefully quicker to be applied.