Rework dump command parsing

Includes better error message and tab completion.

Issue: https://tree.taiga.io/project/jktjkt-netconf-cli/issue/182
Issue: https://tree.taiga.io/project/jktjkt-netconf-cli/issue/183
Change-Id: I6d91870617f99423cc25926522bd1408587c2459
diff --git a/tests/command_completion.cpp b/tests/command_completion.cpp
index 06cedd7..cf6b188 100644
--- a/tests/command_completion.cpp
+++ b/tests/command_completion.cpp
@@ -73,5 +73,12 @@
         expectedContextLength = 0;
     }
 
+    SECTION("dump")
+    {
+        input = "dump ";
+        expectedCompletions = {"xml", "json"};
+        expectedContextLength = 0;
+    }
+
     REQUIRE(parser.completeCommand(input, errorStream) == (Completions{expectedCompletions, expectedContextLength}));
 }