title: How to not exit a click CLI?
body: <p>I'm writing a python script, that should behave like a typical shell and providing some self written functions.
  It is working quite well already, but it always exits after a successful command, so that it has to be started again to perform a second task.
  How can I make it, so it doesn't finish with exit code 0 but returns to shell awaiting new input? How would I have to implement exit methods then?
  Following example always exits after typing print-a or print-b:</p>
  
  <pre><code>import click
  import click_repl
  from prompt_toolkit.history import FileHistory
  import os
  
  @click.group(invoke_without_command=True)
  @click.pass_context
  def cli(ctx):
       if ctx.invoked_subcommand is None:
           ctx.invoke(repl)
  
  @cli.command()
  def print_a():
      print("a")
  
  
  @cli.command()
  def print_b():
      print("b")
  
  
  @cli.command()
  def repl():
      prompt_kwargs = {
          'history': FileHistory(os.path.expanduser('~/.repl_history'))
      }
      click_repl.repl(click.get_current_context(), prompt_kwargs)
  
  def main():
      while True:
          cli(obj={})
  
  
  if __name__ == "__main__":
      main()
  </code></pre>
  
  <p>(And a bonus question: In the cmd package it is possible to customize the <code>&gt;</code> prompt tag, is this possible with click to? So that it's something like <code>App&gt;</code> instead?)</p>