flake8 plugin automatically writes buffer even if not intended

[raven42]

Seeing an issue where when opening a python file from a stream, and if flake8 is set to open by default using other VIM options, then it results in a file being written when it is unintended.

For example:

git show some_branch:./test.py | vim -c "file some_branch:test.py" -c "doautocmd BufRead test.py -"

This reads the contents of a file on another branch and opens it in vim. Then if the following is set in .vimrc, to open flake8 by default on any python file:

autocmd BufEnter *.py call Flake8()

This can be solved by adding an option to disable the auto file write in flake8.

Fixed by #97