Add support for user config file

[achaussier]

Hello,

First, thanks for this useful project I begin to use, with really great features !

I've checked quickly, but I haven't see the management of a user config file (as .cookiecutterrc) to manage user default answers on top of template default answers.

The order of override could be (from top to bottom):

1. Template default values
2. If existing user values (from ~/.copierrc ?)
3. Last used values if already deployed and .copier-answers.yml exists

What do you think about that ?

[yajo]

Absolutely needed!