Version 1.2.8

[endernoke]

This PR mainly unifies the configurations across the app to Config class.
It also introduces minor new commands.

New Commands

• instagram config --reset: reset all configurations to default.
• instagram auth switch-account <username>: Convenience command for switching between different logged-in accounts (so the user doesn't have to type instagram config --set login.current_username <username>). The first command also performs basic input validation.

Code Changes

• instagram.configs.Config is reimplemented as a singleton class. Example usage is as follows:

  # No need to create a new Config instance
  Config().get("chat.layout")  # e.g. returns 'compact'
  Config().set("login.default_username", "george")  # The updated value will be immediately available in the next call to Config() from any interface