Use a more DRY way of handling command line args and flags

[brikis98]

To create or modify a command line argument or flag, you currently have to:

1. Update the relevant Command struct (e.g. DeleteCommand)
2. Update a description variable (e.g. deleteDscrAwsRegion)
3. Update the Help function for the Command struct
4. Update the Synopsis function for the Command struct
5. Update the flagSet defined in the Run function for the Command struct

This is way too much repetition. We should use a DRYer library for handling command line args & flags, such as jessevdk/go-flags.