Allow Python commands to optionally take an SBExecutionContext argument in...

Allow Python commands to optionally take an SBExecutionContext argument in case they need to handle 'where they want to act' separately from the notion of 'currently-selected entity' that is associated to the debugger. Do this in an (hopefully) non-breaking way by running an argcount check before passing in the new argument. Update the test case to also check for this new feature. www update to follow

llvm-svn: 218834