Add support for passing token as argument

[minthemiddle]

We're trying to set up slack-cli as a status updater for all members on our remote team. For this, we need to pass everyone's token to the CLI, preferably via argument.

What we would like to do:

• Set up a crontab manager like Cronkeep on our team server
• Everyone puts her work hours in the crontab
• We save everyone's token in .env, like SLACK_TOKEN_EMAILALIAS
• The command for the auto status update would for example be slack status edit --text="Pomodoro" --emoji=":tomato:" --token="SLACK_TOKEN_TEST"

If there is any chance for this to be implemented, we would work on a PR. Happy for direction where in the script to add the part (At the beginning where you check for the token? Or rather at the command level?)