Finished the unit test of the filename plugin

@klaus

1. Finish the unit test of the filename. To test the FilenameInputPlugin We need to using the other plugin like none-bin and a joinfile(This plugin join the file together, which is a mock object for the WendelinOutputPlugin ). The TestHelper could register all those plugins for tests.
2. Adding the configuration for order_by_modified_time, by default it is zero, we order the file alphabetically. If it equals "1", we order the files by modified ascending order, else we order the files by descending order.
3. Adding the configuration for the multi-directories. configure the multi_dir: ["path/to/first_dir","path/to/second_dir",..], and configure the multi_tag: ["tag1","tag2",...] .the filename plugin could load the files in each directory and sort them separately.