add a way to change environment variables inside the shell

[johnwebbcole]

A common issue I run into:

• Start fuge
• Start all
• Need to change environment var in a service.
• Stop fuge
• Change fuge.yml
• Start fuge
• Start all

It would be nice to be able to change the in-memory copy of a service environment variable inside the shell.

For example:

• Start fuge
• Start all
• set-env myservicename myvar myval
• restart myservicename