Prop consistency

Decide on something final for v2 docs, support old formats for 6 months, then warn, then remove in 2017 :)
- Caps vs Underscore (`LOCALE` vs `_html`)
- Overrides vs Variables (`KEY` vs `NUM`) -- note, messageformat usually uses caps for vars!

```
  {{#mf KEY='there_are_widgets' NUM=getNum}}
```

We should try differentiate between the two more clearly.
