Allow combining themes (output.html.theme and theme/)

### Problem

I'm trying mdBook 0.5.2 with `output.html.theme = "third_party/theme"` and a `theme/` directory and the latter is completely ignored. The latter contains only the favicon.

### Proposed Solution

Allow `theme/` to override `output.html.theme` specified theme.

Allow head.hbs combination as well.

### Notes

- [ ] allow head.hbs combination