support stdin

[dirk-thomas]

If pydocstyle could be invoked with - to indicate to read the content which should be linted from stdin that would enable better integration of the tool in various editors. E.g. on-the-fly linting (with now need to save the file explicitly) in Atom: AtomLinter/linter-pydocstyle#34

[sigmavirus24]

@dirk-thomas flake8-docstrings + flake8 make this possible and easy (for a short-ish term solution).

[milkey-mouse]

Where would pydocstyle look for config files? Would it just use the defaults?

[Nurdok]

It should not load configuration files unless explicitly passed with #117.