dunst,task,zathura: Fix styler uncommitted changes

Prepared dunst config for using the newly introduced (v1.8.0) drop-in
configuration files. This allows spreading out dunst configuration over
arbitrary files in the `dunstrc.d/` folder in the usual configuration
folder. Will be perfect for less intrusive `styler` changes.

Added icons to be displayed on dunst notifications if there are any.

Also prepared zathura configuration in a similar way, with the
configuration file containing an include directive for a colorscheme
file.

Lastly, moved taskwarrior theme options into their own file and included
them with an include directive.
Here, the file itself needs to be committed as well since taskwarrior
will complain about missing file being included otherwise. However, we
simply ignore the file in git after force-including it and no changes
will be tracked.

This means that from now on, styler theme changes should not show up as
uncommitted changes in the repository anymore for currently active
programs.

Fixes #6.

dunst/.config/dunst/dunstrc

@@ -160,10 +160,10 @@
     ### Icons ###
 
     # Align icons left/right/off
-    icon_position = off
+    icon_position = left
 
     # Scale larger icons down to this size, set to 0 to disable
-    max_icon_size = 32
+    max_icon_size = 128
 
     # Paths to default icons.
     icon_path = /usr/share/icons/gnome/16x16/status/:/usr/share/icons/gnome/16x16/devices/
@@ -415,22 +415,3 @@
 #    set_stack_tag = "volume"
 #
 # vim: ft=cfg
-# Base16 bespin - dunst color config
-frame_color = "#8a8986"
-separator_color = "#8a8986"
-
-[base16_low]
-    msg_urgency = low
-    background = "#36312e"
-    foreground = "#666666"
-
-[base16_normal]
-    msg_urgency = normal
-    background = "#5e5d5c"
-    foreground = "#8a8986"
-
-[base16_critical]
-    msg_urgency = critical
-    background = "#cf6a4c"
-    foreground = "#9d9b97"
-# Base16End bespin - dunst color config