Use less generic env variables

[nhoizey]

The environment variable names are really generic:
https://github.com/KyleMit/eleventy-plugin-embed-tweet/blob/master/twitter.js#L56-L69

I have keys for multiple services in my .env file, so it would be better having a more distinctive name for these for Twitter.

Here are the names in Twitter's UI:

I suggest using these names, so follow Twitter naming:

current name	new name
TOKEN	TWITTER_ACCESS_TOKEN
TOKEN_SECRET	TWITTER_ACCESS_TOKEN_SECRET
CONSUMER_KEY	TWITTER_API_KEY
CONSUMER_SECRET	TWITTER_API_SECRET_KEY

If you agree with this, I can make a PR to use new names AND still support current ones for compatibility.

[KyleMit]

I like this change. We'll have to either support the old keys, with priority to new names and then fallback to older names, or revision as a breaking change. Not sure how many people are genuinely using this repo - and it's not a big lift to rename some properties. I wonder if we could code mod them over, but there just doesn't seem to be enough there to want to have users run a script to fix stuff.

[nhoizey]

I think you could support older names for a while, with a message in the console to show they're deprecated.