Added ability to extend exec environment variables

[VojtechBartos]

Hey guys,
i have got to situation when i need to extend exec command environment variables and i don't wanna add it to process.env. I have just added additional _.extend call for options.env.

Take a look and let me know ;)

Vojta

[shaharke]

_.extend can accept multiple objects as sources. Please fix the code to look like this:

_.extend( processOptions.env, options.env, { PYTHONUNBUFFERED: unbuffered } );