Improve settings tests and validation

[liquidnya]

• Change test cases to use settings.js instead of a mock of settings.js by writing the settings to settings.json inside the virtual file system before starting the queue. (already done by Add new save file handling #22)
• Add a validation for channel: This needs to be a valid twitch username consisting of only lowercase letters, numbers, and underlines and have at least 3 letters. This is validated, because the channel is used in the URL in twitch.js and setting something else (e.g. your display name with uppercase letters, or e.g. containing kanji) will not work and the bot will crash at a later time anyways.
• Make it more clear that the lowercase username is needed in the example settings file and remove gender from example. (already done by Add new save file handling #22)