Add way to store credentials/config locally, rather than using env vars every time

[patrickdevivo]

Some functionality, like the GitHub functions, requires a configuration value (GITHUB_TOKEN) set via an env variable. It's a bit clumsy to specify this every time (GITHUB_TOKEN=123 askgit "SELECT ....") and would be great if there were a way to store the token (or any other config value) in a "cache" or managed config area on disk, so that a user doesn't need to specify it on every run of the command