remove commmand settings from config options

removes the following options in favour of the global/project level flake8 config files:

g:flake8_builtins
g:flake8_ignore
g:flake8_max_line_length
g:flake8_max_complexity

I was thinking about adding a depreciation message based on whether the user had the old settings in their vimrc loosely:

```viml
if exists('g:flake8_ignore')
  echom "g:flake8_ignore has been depreciated in favour of flake8 config files"
  echom "http://flake8.readthedocs.org/en/latest/config.html"
endif
```

it would be bulky but after a while you could remove it?
 
ref #33