office: Rename mail module to office module

Reflecting the somewhat expanding scope of the module, renamed it to
office. Still keeps the old files and setups but also got a new README
file.

office/.config/khal/config

@@ -0,0 +1,16 @@
+[calendars]
+
+[[calendar_local]]
+path = ~/documents/calendars/*
+type = discover
+color = auto
+
+[locale]
+timeformat = %H:%M
+dateformat = %Y-%m-%d
+longdateformat = %Y-%m-%d
+datetimeformat = %Y-%m-%d %H:%M
+longdatetimeformat = %Y-%m-%d %H:%M
+
+[default]
+default_calendar = Activities