Add support for the "configure get" subcommand

This allows a user to print a single configuration value
from their AWS config file.  It only looks in the config file, does not
look in env vars, command line args, etc.  It supports scoped values (to
a specific profile) as well as explicitly specifying a section name via
(`section-name.config-value`).

In order to support this, I added support for positional arguments in
the Command class arg table.  I also added support for reading
descriptions/examples from a specific file, rather than relying on the
default value.  This is needed for subcommands, unless we pass through
parent command information to subcommands.
Add support for a "configure set" command

This allows a user to set a single configuration variable
without having to edit the contents of the config file.

This makes it possible to completely shield the user from knowing
where the config file is.

This supports the same syntax as "get" with respect to the qualified
and unqualified names.
Update CHANGELOG with get/set commands
Fix python3 bug regarding arg ordering

Need to ensure args match the same order as the ARG_TABLE
list.
Incorporate review feedback
Move changelog entry to next release
Merge branch 'config-get-set' into develop

* config-get-set:
  Move changelog entry to next release
  Incorporate review feedback
  Fix python3 bug regarding arg ordering
  Update CHANGELOG with get/set commands
  Add support for a "configure set" command
  Add support for the "configure get" subcommand
s/executer/executor
Merge branch 'executor' into develop

* executor:
  s/executer/executor