Feature/support swedish language

[MariaSL]

Description

This PR enables generating data for both English & Swedish languages (and potentially other languages).

Major changes:

• Introduced a language parameter to scripts thats parse wikitext, build Wiki DB, process html, build Relation DB, build Alias DB, annotate;
• Added scripts for coreference resolution and nel;
• Moved several functions (extract_redirect, generate_instances) to utils;
• Updated requirements;
• Updated README with step-by-step explanation of data generation process.

All tests are passing

[rloganiv]

Hi @MariaSL,

This looks fantastic! Thank you so much for pushing these changes - in addition to the support for multiple languages, it also is much cleaner!

In order to merge this work into the master branch, I need to ensure that the English Linked WikiText-2 dataset can still be reproduced. I notice some constants have been replaced/introduced which only work for Swedish (e.g. the list of months in render.py). Can you enable "add edits from maintainers" so I can address these issues / add some more tests?