feat: add Instagram provider

[justlevine]

What

Adds support for Instagram as an OAuth2 Provider, using https://github.com/thephpleague/oauth2-instagram.

Why

closes #10

How

• feat: Add Instagram provider support.
• dev!: Move loginOptions.linkExistingUsers to {Provider}LoginOptions.

Testing Instructions

Additional Info

linkExistingUsers is no longer a required login option, as Instagram's OAuth doesnt return enough information (e.g. email) to auto-link.

Checklist:

• [x] My code is tested to the best of my abilities.
• [x] My code follows the WordPress Coding Standards.
• [x] My code has proper inline documentation.
• [x] I have added unit tests to verify the code works as intended.
• [x] I included the relevant changes in CHANGELOG.md

[coveralls]

Coverage increased (+0.2%) to 68.601% when pulling 9a7a0b6a1ac1dfc1bc01721fa58f6a4f1e5be8a1 on justlevine:feat/add-instagram-provider into 57d6d2158e758d8b45b4305497611b3880a72349 on AxeWP:develop.