Allow the use of global config file

If not found elsewhere, look for the config file in /etc/todo/config.
`make install` installs a global config file, setting TODO_DIR=~/.todo.
todo.sh creates TODO_DIR if it doesn't exist.

tests/t0000-config.sh

@@ -8,6 +8,10 @@ to find it somewhere else.
 '
 . ./test-lib.sh
 
+
+# Override default global config file
+export TODOTXT_GLOBAL_CFG_FILE=global.cfg
+
 # Remove the pre-created todo.cfg to test behavior in its absence
 rm -f todo.cfg
 echo "Fatal Error: Cannot read configuration file $HOME/.todo/config" > expect
@@ -55,6 +59,14 @@ test_expect_success 'config file (default location 3)' '
         rm -f .todo.cfg
 '
 
+rm -f used_config
+test_expect_success 'config file (global config file)' '
+    cp test.cfg "$TODOTXT_GLOBAL_CFG_FILE"
+    todo.sh > output;
+    test_cmp expect output && test -f used_config &&
+        rm -f "$TODOTXT_GLOBAL_CFG_FILE"
+'
+
 rm -f used_config
 test_expect_success 'config file (command line)' '
     todo.sh -d test.cfg > output;