Customize templates location and use different theme

Hello,

On my way to use our own template with Pirum, I've first created Pirus [1]. 
Now as I don't want to follow Pirum updates, I propose this PR with its explain at [2] 

Tell me if you think that this request could be include or not in Pirum.

Regards

[1] https://github.com/llaville/Pirus 
[2] http://php5.laurent-laville.org/pirus/pirum.html