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/export.txt

@@ -0,0 +1,22 @@
+Export
+======
+
+Export calcurse data (appointments, events and todos).
+This leads to the export submenu, from which you can choose between
+two different export formats: 'ical' and 'pcal'. Choosing one of
+those formats lets you export calcurse data to icalendar or pcal
+format.
+
+You first need to specify the file to which the data will be exported.
+By default, this file is:
+
+     ~/calcurse.ics
+
+for an ical export, and:
+
+     ~/calcurse.txt
+
+for a pcal export.
+
+Calcurse data are exported in the following order:
+     events, appointments, todos.