Econll

- added `stats` for computing data statistics
- added `stat` command to CLI
- added linter comments to `converter` and `tabler`

- refactored `convert`
- removed `labels` from `refs`

- added CLI interface to `convert`
- added reference label input for `convert` (reusing `refs`)
- added JSONL, YAML, & LIST rile reading to `__main__`

- added conversion (`convert`) to/from CoNLL data (`conll`):
- YAML/Markdown format (`mdown`)
- JSON/JSONL parse (`parse`)

- added support for WordPiece tokenization indexing

- added `xcoder` functions and tests