text: Title: Disable welcome message
  Body: I'm working on migrating my `.xonshrc` into a xontrib library, but after I've done that, I get the welcome message, which says "Create ~/.xonshrc file manually or use xonfig to suppress the welcome message".
  
  How does one "use xonfig to suppress the welcome message"?
  
  I can't find anything in the docs or the xonfig help that suggests how to suppress the welcome message. Moreover, when I look at the code, I don't see any hooks that could plausibly work to suppress the message.
  
  It looks like I can suppress it by `monkeypatching xonsh.main.print_welcome_message`.
  
  Or by setting `xonsh.xonfig.WELCOME_MSG = []`.
  
  It looks like I can do it with one line and avoiding polluting the namespace with `__import__('xonsh.xonfig').xonfig.WELCOME_MSG = []`. Is that the recommended syntax? Should there not perhaps be a cleaner way to do that and for the welcome message to make it easier to provide or link to that technique?