rewritten plugins paths management

now paths are managed by FFPlugins itself. parse_input_args adds paths
to it, find and find_all do not touch this anymore. Is cleaner and safer
:)
FFPlugins help is now managed in FFPlugins instead of external function
magic pattern cleanups: helper is called only if pattern is specified

also rename _parse_input_args__prepare_anon_pattern ->
_parse_input_args__prepare_magic_pattern
added documentation for _parse_input_args__prepare_magic_pattern
simplified test for delimiters in magic pattern
editorial in documentation
lost plugins_paths deletion
documentation
move regexp outside function called many times
outsource processing single source of main
docstring for is_path_excluded