Feature request: merge keys from multiple config.yml files instead of overriding

**Is your feature request related to a problem? Please describe.**
I have "public" and "private" config.yml files (one in my dotfiles repo and one in a private repo). If I define a `customCommands` key in both of them, it seems like the one in the second file overwrites the one in the first completely, even if the commands aren't redefined.

**Describe the solution you'd like**
I would like it if the config keys were merged, overwriting only matching subkeys (i.e. if I define the same command twice, the one loaded first should be overwritten)

Hope I'm not missing something here. I have this:
```yaml
export LG_CONFIG_FILE="$HOME/.config/lazygit/config.yml,$HOME/.config/lazygit/config_priv.yml"
```
and if I define `customCommands` in both, I lose any custom commands I defined in the first file.