Trailing commas for match block arms

Attempt to implement an option for trailing commas for block based match arms (issue
173). Put in place test files to verify this behaviour.

tests/source/match-block-trailing-comma.rs

@@ -0,0 +1,13 @@
+// rustfmt-match_block_trailing_comma: true
+// Match expressions, no unwrapping of block arms or wrapping of multiline
+// expressions.
+
+fn foo() {
+    match x {
+        a => {
+            "line1";
+            "line2"
+        }
+        b => "bar",
+    }
+}