Set up more reliable battery threshold

Turns out low_threshold is in minutes, not in percent, by default.
Needless to say, time estimates are grossly unreliable here and 10
minutes left is too late anyway.

dotfiles/i3status.conf

        status_bat = "⚡B0"
        status_full = "☻F0"
        path = "/sys/class/power_supply/BAT%d/uevent"
-       low_threshold = 10
+       low_threshold = 30
+       threshold_type = percentage
        hide_seconds = true
 }