Add support for "config get/set" command

**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 "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.