Add support for custom formats with labels

Hi! I started switching to Waybar at the weekend and found this to replace what I had for Polybar previously. It works great—thanks for merging #29 so quickly btw—but ideally ***I'd like to show counts separately***. I couldn't find a way to get Waybar passing anything from the "real" `format` option in the bar config, so instead I've hacked together some basic "label" parsing. It supports a custom prefix/suffix (separated by `:`) which allows for per-count auto-hiding icons, but at the cost of a *really* ugly sed block. ## New CLI args * `-f, --format` * `-t, --tooltip` ## Supported Labels * `{aur}` * `{dev}` * `{pacman}` * `{total}` ## Examples ### New defaults ```jsonc "exec": "waybar-updates", // All examples are based on this: "format": "{icon}{}", "format-icons": { "pending-updates": " ", "updated": "" } ```  ### Custom prefix and suffix ```jsonc "exec": "waybar-updates --format '{total}{ (+:aur: AUR)}'", ```  ### Multiline Pacman+AUR example with icons ```jsonc "exec": "waybar-updates -df '{ :pacman:\n }{ :aur}'", ```  ### Showing only package counts in the tooltip ```jsonc "exec": "waybar-updates --tooltip '{ :pacman}{  :aur}{  :dev}'", ```  ### Showing all counts individually (and hiding them in the tooltip) ```jsonc "exec": "waybar-updates -df '{pacman}{  :aur} {  :dev}' -t '{}'", ```