Module aliases, and the module docs formatter showing In core vs extras, etc

[Michael DeHaan]

As requested by Monty and Eric, among others,

I'm proposing the following on the development branch:

- the module docs formatter shows "core" vs "extras" modules in the index in different columns.

- the plugin loader learns about "aliases", so renamed modules can be found even though the module names can change - enabling us to rename some modules.

- we teach the plugin loader to ignore a module starting with "_" and use this to indicate a module is deprecated.

This might allow us to show them in a seperate list, and occasionally replace a module with a new module, while still leaving the old one around for several releases.

This might cause the module output to look like:

<CATEGORY>

Core Modules        Extras Modules        Deprecated Modules (if any)

=====                    =====                      ======

Basically in a three column view.

The actual alias mapping might have to be hard coded in the main ansible project, because it spans repos.