[__sensible_editor] Add exists as a possible --state value

cdist/conf/type/__sensible_editor/man.rst

@@ -24,7 +24,15 @@ editor
 OPTIONAL PARAMETERS
 -------------------
 state
-    Either "present" or "absent". Defaults to "present".
+    'present', 'absent', or 'exists'. Defaults to 'present', where:
+
+    present
+        the sensible-editor is exactly what is specified in --editor.
+    absent
+        no sensible-editor configuration is present.
+    exists
+        the sensible-editor will be set to what is specified in --editor,
+        unless there already is a configuration on the target system.
 
 
 EXAMPLES

cdist/conf/type/__sensible_editor/manifest

@@ -47,9 +47,9 @@ os_version=$(cat "${__global}/explorer/os_version")
 state=$(cat "${__object}/parameter/state")
 user=$__object_id
 
-if test "${state}" != 'present' && test "${state}" != 'absent'
+if test "${state}" != 'present' && test "${state}" != 'exists' && test "${state}" != 'absent'
 then
-	echo 'Only "present" and "absent" are allowed for --state' >&2
+	echo 'Only "present", "exists", and "absent" are allowed for --state' >&2
 	exit 1
 fi