Unify and customizable Conll-like readers

The current ConllU and ontonotes reader are both in the same format, the conll-like readers. There are many variations to the CoNLL format, such as different fields for different columns. The current readers are hard-coded on the fields, and the file extensions. However, one single reader should be able to handle these, with some configurable file extensions and field names.

Start by locating the current CoNLL-U and OntoNotes readers and comparing their hard-coded fields and file extensions. Done should mean one configurable CoNLL-like reader can handle both formats and support variations through configurable extensions and field names.

Written by the indexing model from the issue text.