Adding an environment variable similar to TF_VERSION

[bjbishop]

Hey Warren, great tools! I use them a lot

I noticed that tfswitch can use an env variable TF_VERSION to automatically switch to that version. Can you add the same for tgswitch? Something like TG_VERSION?

I use direnv (https://direnv.net/) a lot and it's handy to include a .envrc file in a directory to switch things around, and I could include TF_VERSION and TG_VERSION variables in it to use unique tools per environment.

[bjbishop]

I think 0.6.0 now closes this, thank you @warrensbox !!!