Better Kotlin support: LayoutContainer, synthetics

Current documentation says that custom kotlin models are not officially supported.

I'm wondering if `EpoxyModel` augmented with `LayoutContainer` will let to skip the holder completely (https://kotlinlang.org/docs/tutorials/android-plugin.html#layoutcontainer-support). For some reason, at the moment it doesn't. Here is my attempt to make it work: https://github.com/Ghedeon/Trendroid/blob/master/app/src/main/java/com/ghedeon/trendroid/ui/trending/RepoItem.kt

As you can see, I had to disable the caching (`@ContainerOptions(cache = NO_CACHE)`) because my items were not rendered properly. And without cache, it's just a regular `findViewById`. The concise syntax is pretty neat tho...

Some possible (?) options with `Synthetics + EpoxyModel`:

1. `LayoutContainer` with no cache. Cleaner version of `EpoxyModel + findViewById` (slow).
2. `LayoutContainer` + Holder. Cleaner version of `EpoxyModel + KotlinEpoxyHolder` (better)
3. `LayoutContainer` with cache enabled. Supposedly solves all the above without the need of `Holder`.

Start by reading the current documentation on custom Kotlin models and the linked RepoItem.kt example, then compare the three proposed LayoutContainer and synthetics options. Reproduce the rendering behavior with caching enabled and disabled; done requires a decided support approach with its expected behavior documented.

Written by the indexing model from the issue text.