Run fixer before saving a file

[camagu]

ale runs the fixer after the file has been saved which means saving the file
twice. This is specially annoying when reacting to file changes outside of vim.
Fixers should be run before the file has been saved.

[camagu]

ale fixers are run async, so using an autocommand to run the fixers before
saving does not work as intended. There are two options: stick to
g:ale_fix_on_save or bind ALEFix fix the code on demand.