It should be possible to select no errors

[Nurdok]

This might sound a bit silly, but I came across this when trying to integrate pep257 to a build system of a big repo at work. I wanted to have pep257 run for every python file, but to manually and selectively include more and more checks, to specific areas of the code. For that, it's necessary to have a default config file that doesn't check for errors.

This is possible today, by ignoreing all the error codes, but this is problematic as you need to manually write all the error codes. Also, when new error codes are added, one needs to add them to the ignore list.

[Nurdok]

Fixed by #135