Allow the env:UserInstallation variable to be specified when launching a listener

It seems that the way to allow for multiple parallel instances of unoconv is to have each instance [set its own value for `env:UserInstallation`](http://ask.libreoffice.org/en/question/14841/how-can-i-use-soffice-from-the-command-line-when-quickstarter-is-running/?answer=29735#post-id-29735), which `soffice` takes as a parameter on initialization (setting a different port per listener is most likely required too, but that's already available).

This PR adds an -E / --environment switch to set this variable. 

I don't really have python experience, so I apologize if there is a better way to code this (I'm a ruby programmer :smile:).