Export online help as text files

Move the texts of each online help screen to a separate text file. The
set of files is supposed to replace the online help screens later.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

doc/priority.txt

@@ -0,0 +1,17 @@
+Priority
+========
+
+Change the priority of the currently selected item in the TODO list.
+Priorities are represented by the number appearing in front of the
+todo description. This number goes from 9 for the lowest priority to
+1 for the highest priority.
+Todo having higher priorities are placed first (at the top) inside the
+todo panel.
+
+If you want to raise the priority of a todo item, you need to press '+'.
+In doing so, the number in front of this item will decrease, meaning its
+priority increases. The item position inside the todo panel may change,
+depending on the priority of the items above it.
+
+At the opposite, to lower a todo priority, press '-'. The todo position
+may also change depending on the priority of the items below.