Sort fixers

Inside Fixer class:
- do not sort fixers two times
- sort not only by priority, but also by level and names

Inside FixCommand class:
- add info of fixer priority to fixers help

Why I propose that changes?
- Sometimes after adding new fixer or rebuild readme on different env order of sorted fixers was differ (shuffle fixers with same priority) and then README.rst was totally reorganized. The diff was awful.
- At start I had no idea what is the order of fixers in readme file. Now it will be clear for any new one in project.
People reading the README do not care about the priority (this is really just an implementation detail), I think it should be removed here.