[PATCH 1 of 6] extensions: import the exthelper class from evolve ff799015d62e

[Yuya Nishihara]

On Wed, 05 Dec 2018 00:08:24 -0500, Matt Harbison wrote:
> Does the name seem reasonable with this wider audience?  I wanted to put  
> it inside the extensions module, but it looks like there's a cycle around  
> commands.py.

I'm okay with the name exthelper as it is a utility-level function built
on top of core extension API. In order to resolve cycle, commands.table
could be passed in to the exthelper as an argument.