Add TaigaControl class

This is in preparation for having configurable controls in a Taiga
based game via an INI file.
Move extern declaration of 'key' and 'mouse' to TaigaInput.cpp

These objects are already have extern declarations in Taiga.hpp and
are not needed elsewhere.
Rename TaigaControl to TaigaKeybind

This name is a little bit more more accurate as to what the purpose of
the class is.
Add 'none' option to TaigaKeybindType enum

This allows the option of having an action for which there is no
control bound to it.
Add default constructor for TaigaKeybind

This will be necessary for when support for reading key bindings out
of an INI is added.
Add (in)equality operators for TaigaKeybind
Add the TaigaConfig class

This class provides a simple and easy interface for Allegro's built-in
configuration file support, and even automatically translates string
"keys" into the proper type.
Automatically mount game's base directory as readable and writable.