Escape MetaString values (as added with --metadata flag).

Previously they would be transmitted to the template without
any escaping.

Note that `--M title='*foo*'` yields a different result from

    ---
    title: *foo*
    ---

In the latter case, we have emphasis; in the former case, just
a string with literal asterisks (which will be escaped
in formats, like Markdown, that require it).

Closes #3792.

1 files changed, 13 insertions, 0 deletions

diff --git a/test/command/3792.md b/test/command/3792.md
new file mode 100644
index 000000000..7c76afee7
--- /dev/null
+++ b/test/command/3792.md
@@ -0,0 +1,13 @@
+Make sure metadata values are treated as strings,
+and properly escaped.
+
+```
+% pandoc -t markdown -s -M title="<this> *that*"
+ok
+^D
+---
+title: '\<this\> \*that\*'
+---
+
+ok
+```