Implement tapegen help

[MarcelGarus]

• provide a general help screen
• provide help for init
• provide help for assist
• provide help for doctor
• provide help for help (easteregg opportunity)

[MarcelGarus]

General help output:

Welcome to tape!
For information about what tape is and how to use it, consult https://pub.dev/packages/tape
This help page will only focus on the command line tool.

Usage:
  pub run tape <command> [<options>]

Commands:
  help       displays help
  init       create tape boilerplate for your project
  assist     assists you while writing code
  doctor     information about the usage of tape in your project

Global options:
  --simple-ouptut   for output without ansi colors or replacement

More help: tape help <command>
Questions? Issues? https://github.com/marcelgarus/tape/issues/new