Override composer task parameters from env (stage | production) config file

[t0k4rt]

Hi,
I tried to fix some issue I had when working with composer capistrano plugin.
It' not currently possible to override task parameters from config file, feature that would be very convenient. I made a fix and a pull request.

Some people had the same kind of issue: #39 #22
Feedback would be appriciated, see #26