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

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@218834 91177308-0d34-0410-b5e6-96231b3b80d8