Fixed YAML metadata with "chomp" (|-).

Previously if a YAML block under `|-` contained
a blank line, pandoc would not parse it as metadata.

test/command/yaml-with-chomp.md

@@ -0,0 +1,12 @@
+```
+% pandoc -s -t native
+---
+ml: |-
+    TEST
+
+    BLOCK
+...
+^D
+Pandoc (Meta {unMeta = fromList [("ml",MetaBlocks [Para [Str "TEST"],Plain [Str "BLOCK"]])]})
+[]
+```