Add layout for horizontal main panel

**Is your feature request related to a problem? Please describe.**

I would be nice to use a side-by-side diff in the main panel, but it's too narrow in many cases.
In addition, it's easy to scroll up and down in the main panel, but line wrapping is more difficult to deal with.

**Describe the solution you'd like**

I'd like to switch layouts (preferably using config.yml) to have the main panel be horizontal across the bottom of the window.

**Describe alternatives you've considered**

One alternative would be having a keybinding that would "zoom in" on the main panel, allowing it to take up most/all of the window and optionally adding a `--side-by-side` flag to the pager command.
The main drawback with this solution is that it would make navigation to different files more tedious.

**Additional context**

```
_____________________________________________________________________
| -- Status ----------- -- Files ------------ -- Commits ---------- |
| | | | | | | |
| --------------------- | | | | |
| -- Branches --------- | | | | |
| | | | | | | |
| | | | | --------------------- |
| | | | | -- Stash ------------ |
| | | | | | | |
| --------------------- --------------------- --------------------- |
| -- Unstage Changes ---------------------------------------------- |
| | | |
| | 1 foo | 1 bar | |
| | 2 | 2 | |
| | 3 baz | 3 baz | |
| | 4 | 4 | |
| | | |
| ----------------------------------------------------------------- |
_____________________________________________________________________
```