Add discard command

Change-Id: Ie2243188694657665240769d93b2835034e48c3a
diff --git a/src/interpreter.cpp b/src/interpreter.cpp
index bdeecf9..452bc74 100644
--- a/src/interpreter.cpp
+++ b/src/interpreter.cpp
@@ -29,6 +29,11 @@
     m_datastore.commitChanges();
 }
 
+void Interpreter::operator()(const discard_&) const
+{
+    m_datastore.discardChanges();
+}
+
 void Interpreter::operator()(const set_& set) const
 {
     m_datastore.setLeaf(absolutePathFromCommand(set), set.m_data);