Use `-C` / `--extra-config` to load Lua files to overwrite the default
or user defined config.
This helps with integration, where integrating xplr with another tool
requires xplr to overwrite some config, without requiring the users to
install an xplr plugin or update the xplr config.
Example:
```bash
xplr -C one.lua two.lua
# Or
xplr -C one.lua -C two.lua
```
> **WARNING:**
>
> Extra config doesn't require specifying the `version`, hence, it's the
> integration author or the user's responsibility to assert
> compatibility using the globally exposed `version` in the extra config
> files, similar to xplr plugins.
Ref: https://github.com/sayanarijit/xplr/issues/316
