Add Filter [Init] section

As discussed in #108, this change allows addition of an `[Init]` section, similar to _action_ configs, to _filter_ configs. This allows standard options for filters to be set, currently only _maxlines_ for multi-line regexs, but also overridden from within _jail.conf_.

I've broken out most the logic from `ActionReader` to a new `OptionConfigReader` class (maybe a better name for it?), which is then inherited by `ActionReader` and `FilterReader`.