Covers the use case where needing to exclude more than 1 output but
still include all other displays.
e.g. I have 3 monitors: laptop + HD + 4K; and 3 bar types:
- The main bar is on the laptop. `output: "laptop-monitor-id"`
- The 4K has a specific waybar bar-1 configuration. `output: "4K-monitor-id"`
- I want all other displays (3rd HD monitor / any HDMI output when presenting)
to have a plain bar: `output: ["!laptop-monitor-id", "!4k-monitor-id", "*"]`
This commit adds support to reading the config base path from the
environment variable `WAYBAR_CONFIG_DIR`. If it is set, but no
configuration is found there, it falls back to the previous mechanism
of using the default paths, without erroring.
