add and detect a configuration file

A configuration file can define some parameter of the platform or the project structure. Such that DESIGN_DIR is a variable pointing to /platform/designs or another folder.

This would allow vpm to adapt to more project structure.

The configuration can also specify some parameters dedicated on the behaviour of vpm, or override the list of repositories

[x] add config files
[x] support for repositories list override
[x] variables to specify project structure

LudwigCRON / vpm

add and detect a configuration file #2