Merge branch 'pep8-stdin-fix' into 'master'

Cache stdin for Flake8 plugins

Currently plugins (e.g., flake8-docstrings) are struggling to support
users passing data in on stdin (e.g., cat file.py | flake8 -). Until
pep8 fixes this itself, we can monkey-patch its `stdin_get_value`
function to handle the caching for us.

Closes #105

See merge request !44