Add help command

This adds a "help" command to the list of available command mode
commands. You can currently type "help", followed by a topic like "add".
calcurse will then try to open a file named "add.txt" in the
documentation directory and display it in an external pager.

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

src/Makefile.am

@@ -2,6 +2,7 @@ AUTOMAKE_OPTIONS = foreign
 
 bin_PROGRAMS = calcurse
 
+AM_CPPFLAGS = -DDOCDIR=\"@docdir@\"
 AM_CFLAGS = -std=c99 -pedantic -D_POSIX_C_SOURCE=200809L
 
 calcurse_SOURCES = \