Add Support for Twitch usernames when starting the backend

[fpruitt]

Purpose

Ideally, the 'Hello World' example for Twitch Extensions would match the patterns established in the Development Rig. This brings the initial startup in line with starting the Dev Rig by allowing users to specify their twitch username instead of having to grab the related ID's manually.

What Does This Do?

• Adds a function that takes a given username and attempts to find the matching owner_Id
• Adds usage of the function, still exiting if an owner_Id cannot be found
• Maintains backwards compatibility for users still specifying owner_Id in env vars or on the command line.
• Updates the README, removing copy about needing to manually lookup owner_Id.
• Moves the line about needing an SSL cert to the top, so the user knows earlier to generate those (this temporarily tripped me up).

Future Work

I would like to write tests for my additions, but it looks like there aren't any setup just yet and I assume the main maintainer would like to establish that pattern.

[KarbonDallas]

This is definitely something we need to do! Let's work on making this happen.

[fpruitt]

@Amorelandra fixed the spelling issue!