Some fmt fixes

#4 was pretty interesting: command from http://stackoverflow.com/questions/10969366/vim-automatically-formatting-golang-source-code-when-saving was intended for vimrc, so every go file opened after it was autoformatted. Thus, gofmt was executed (index number of go file in session - 1) times.

Now tabwidth is selected from variables in such order shiftwidth -> softtabstop -> tabstop. It is also evaluated every time GoFormat starts, not during script initialization.