Merge branch 'bug/69' into 'master'

Handle EPIPE IOErrors when using more than 1 job

If someone is using flake8 and piping it to a command like `head`, the
command they are piping flake8's output too may close the pipe earlier
than flake8 expects. To avoid extraneous exception output being printed,
we now catch IOErrors and check their errnos to ensure they're something
we know we can ignore.

This also provides flexibility to add further errnos for ignoring on a
case-by-case basis.

Closes #69

See merge request !35