Allow users to customize the default configuration

[pfrenssen]

The current lineup of Chromecast devices have different capabilities. It would be great to allow people to customize the defaults for chromecastize to match their own device.

For example users with a Chromecast Ultra might want to always encode using h265, while users with a 3rd generation Chromecast might want to change the h264 level to 4.2.

We can check if there is a config.sh file in the users' config folder and source it if present. In order to inform users about the possible configuration options and suggest sane defaults for the different devices we can also ship an example file.