Fill empty guide (XEPG) slots with dummy 'unknown' entries

[winguru]

Linking to xteve-project#161 to attempt a possible solution / workaround.

[winguru]

Initial thoughts are to add an option to Filter Groups to apply a dummy xTeVe EPG template to all channels that don't have a matching EPG. This should be flexible enough to customize the dummy data for certain programs without relying on just one default global setting.

[winguru]

I've got the basic framework and interface fleshed out, just need to finish the back-end logic.

[winguru]

Experimental branch is working with all features. Please feel free to test.

Created a basic docker image for testing features:
https://hub.docker.com/repository/docker/winguru/xteve-experimental