Qtwirl

Changs from the previous release: ([diff](https://github.com/alphatwirl/qtwirl/compare/v0.04.0...v0.05.0))
- changed the minimum required versoin of AlphaTwirl to 0.20.0
- renamed the 1st argument of `qtwirl()` `file` `data`
- updated `reader_cfg`
- now `reader` can be a list of readers
- changes in `table_cfg`
- renamed keys
- `keyAttrNames` -> `key_name`
- `keyIndices` -> `key_index`
- `binnings` -> `key_binning`
- `keyOutColumnNames` -> `key_out_name`
- `valAttrNames` -> `val_name`
- `valIndices` -> `val_index`
- `summaryClass` -> `agg_class`
- `summaryColumnNames` -> `agg_name`
- added new keys: `store_file`, `file_path`, `file_name`,
`file_dir`
- changes in `selection_cfg`
- now the full form is `dict(condition=condition)`
- added new keys: `store_file`, `file_path`, `file_name`,
`file_dir`, `count`
- added option to make cutflow table
- added option to store results to files
- reorganized directory structure, created new private sub-packages
`_parser` and `_builder`
- added ``pytest.ini`` file

Changs from the previous release: ([diff](https://github.com/alphatwirl/qtwirl/compare/v0.03.4...v0.04.0))
- changed options to `qtwirl()`
- no longer necessary to give `"qtwirl"` to `user_modules`
- default of `dispatcher_options` is now `None` not an empty dict
(was mutable default)
- default of `user_modules` is now `None` not an empty tuple
- refactored code in main.py
- added a check for empty input (e.g., all files are zombie)
- return a list of empty data frames (was raising exception)
- updated README.md
- added CHANGELOG.md

Changs from the previous release: ([diff](https://github.com/alphatwirl/qtwirl/compare/v0.03.3...v0.03.4))
- added new option `skip_error_files` to `qtwirl()
- changed the minimum required versoin of AlphaTwirl to 0.18.5