yaml output lacks trailing newline

[durin42]

Consider the input

let foo = {
    bar = "baz",
};

out yaml foo;

ucg build emits this, expressed as a diff against /dev/null:

--- /dev/null   2019-09-19 13:50:39.505277628 -0400
+++ tmp.yaml    2019-09-19 16:42:15.955270274 -0400
@@ -0,0 +1,2 @@
+---
+bar: baz
\ No newline at end of file

It feels like ucg should include the trailing newline so that diffing generated configs is a little prettier.