allow envs to drive the auth

[gedw99]

suggest that this be changed so that the env variables can drive it.
then people don't have to put them credentials on disk.

main.go:

os.Setenv("SPACE_TRACK_USERNAME", "username")
	os.Setenv("SPACE_TRACK_PASSWORD", "password")
	os.Setenv("N2YO_API_KEY", "api-key")
	cli.SatIntel()

then people can set it this way.

# https://www.space-track.org/auth/login
export SPACE_TRACK_USERNAME=xxx
export SPACE_TRACK_PASSWORD=yyy

# https://www.n2yo.com/
export "N2YO_API_KEY=zzz

[christopherwoodall]

The linked PR avoids writing creds to disk. When not present, the user is prompted to enter them; they only exist in memory during the life of the process.