install.py: Waterfox compatibility and default to firefox user.js always (#485) * install.py: Integrate waterfox and default to firefox user.js always This commit: - Removes the old --zen switch - Adds a new --browser/-b switch to specify the browser. Available options: firefox, waterfox, zen - Now defaults to the firefox user.js even if there's new user.js added * install.py: Remove the redundant --zen flag