Allow Specifying format when using tee

[asottile]

In GitLab by @agrothberg on Jul 31, 2018, 15:35

I am using flake8-formatter-junit-xml (with --format junit-xml and a --output-file specified) in order to get junit xml formatted xml output from my flake8 runs. I would also like to see the results on the screen, so I run with --tee. Unfortunately in this case, xml is printed to the screen. Ideally I would be able to control the formatter used for screen output so that it is human readable.