Allow `include` key to have yaml sequence

`include` key can be either yaml sequence or yaml map.

This lets me replace:

```yaml
include:
  smlib.traceback: True
  rattr: True
```

with the (IMHO) more natural:

```yaml
include:
  - smlib.traceback
  - rattr
```

You probably want a test for this and I am a big TDD fan, but I had a little trouble figuring out how to do the test. Feel free to give me some guidance on how you'd like to do it; or if it's quicker for you to add the test yourself, that's cool too.