Add command `clear` to remove all content in stdout

[ericzhang6222]

Add a command clear in arr.ai shell which can clean out stdout like the command clear in Linux. For example:

@> model.app('PetShopApi').package
2020-07-22T14:15:29.20413+08:00 error_message=Missing attr "app" (available: |apps|)

.:1:1:
model.app('PetShopApi').package INFO 
@> model.apps('PetShopApi').package
2020-07-22T14:15:33.980039+08:00 error_message=Missing attr "package" (available: |name,types,endpoints,source_context,attrs|)

.:1:1:
model.apps('PetShopApi').package INFO 
@> model.apps('PetShopApi')
@> clear

will get

@> 

[marcelocantos]

Why is this needed?