text: Title: xontrib load: add option to skip not installed xontribs
  Body: It will be cool to have an ability to load xontribs without errors about not installed ones.
  
  For example this useful in case when you run xonsh in virtual environment and xonsh tries to run ~/.xonshrc and because of the virtual env has no xontribs xonsh produces errors. Using skip missing xontribs option in xonshrc you can prevent errors. 
  
  ## Current Behavior
  
  Show error and return code 1:
  
  ```xsh
  xontrib load whole_word_jumping xontrib_not_installed
  # The following xontribs are enabled but not installed:
  #   ['xontrib_not_installed']
  # Please make sure that they are installed correctly by checking https://xonsh.github.io/awesome-xontribs/
  # Return code 1
  ```
  
  ## Expected Behavior
  Add an option `-s` ("(S)ilent" or "(S)kip missing") and return zero code without errors:
  ```xsh
  xontrib load -s whole_word_jumping xontrib_not_installed
  # Return code 0
  ```
  
  ## Current workaround
  
  [xontrib-rc-awesome](https://github.com/anki-code/xontrib-rc-awesome/blob/5105050cf2f7318eb8b011f1f7a5897574e6ca05/xontrib/rc_awesome.xsh#L68-L83) has this workaround:
  
  ```xsh
  from xonsh.xontribs import get_xontribs 
  _xontribs_installed = set(get_xontribs().keys()) 
  _xontribs_to_load = ('back2dir', 'prompt_bar', 'pipeliner', 'sh',)
  xontrib load @(_xontribs_installed.intersection(_xontribs_to_load))
  ```
  
  ## Start point
  
  https://github.com/xonsh/xonsh/blob/3716f3da8636e41482358b61779e274aade8ecf7/xonsh/xontribs.py#L198
  
  ## For community
  ⬇️  **Please click the 👍 reaction instead of leaving a `+1` or 👍  comment**
  