Added Token Profiles

[WillFantom]

This PR adds in profiles that allow for more accounts to defined in the configuration simultaneously! I imagine this is not a common use case, but it was noted in #70 so I gather I am not alone...

The profile can be set in the configuration file itself (now $HOME/.config/dbxcli.json) with the profile key, or it can be set via flags: --profile <name>. It defaults to the profile default.

To make future changes to the app configuration easier, I have also rewritten some configuration aspects using Viper. Using this, the app will use the configuration path noted above by default, yet if a dbxcli.json is found in the local dir or /config/, that will be used.

[WillFantom]

Bumped to Go version 1.16.x to allow CI to run successfully

[WillFantom]

This also fixes the env default value mistake (I think) that can be found here

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