Read settings from a config file

Source ~/.config/update-notify/config to override THRESHOLD, AUR_REPO
and KERNELS, so settings survive package upgrades. The threshold
argument still takes precedence, so drop it from the service. Add a
commented sample config.

config.example

@@ -0,0 +1,19 @@
+# update-notify configuration, sourced as bash.
+# Copy to ~/.config/update-notify/config and uncomment what you want to change.
+# The values shown are the defaults.
+
+# Minimum number of updates (repo + AUR) before update counts are shown.
+# The command-line argument takes precedence.
+#THRESHOLD=30
+
+# Name of the local aurutils repository.
+#AUR_REPO="aur-local"
+
+# Stock kernel package -> your custom build of it. Replaces the whole list;
+# kernels where either package isn't installed are skipped.
+# Set KERNELS=() to disable the kernel check.
+#KERNELS=(
+#    [linux]="linux-custom"
+#    [linux-lts]="linux-lts-custom"
+#    [linux-zen]="linux-zen-custom"
+#)