Add a --help (-h) option to "command script add" that enables users to define a one-liner short help for their command

Also, in case they don't define any, change the default from "Run Python function <blah>" into "For more information run help <blah>"

The core issue here is that Python only allows one docstring per function, so we can't really attach both a short and a long help to the same command easily
There are alternatives but this is not a pressing enough concern to go through the motions quite yet

Fixes rdar://18322737



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@217795 91177308-0d34-0410-b5e6-96231b3b80d8