Output format bugfixes

Here's a new pull request for two things that have been bothering me:
- 8e4849da6e730e3bae118104ef024e26617e3d39 fixes a bug where the -of parameter's value is incorrectly added to the file list, generated an "Ignoring file" message in the output
- b644f3a825807b754f38f1008e45d092ca2e5314 changes the way the output format is processed so that {4} only contains the number of the character where the error occurs, and not " at character x", which made it impossible for me to get properly formatted messages for Visual Studio. Note that I kept the default behaviour: if the output format has not been customised, it still outputs " at character x", as it did before.