Use lazy front-matter regex

Using a greedy regex could lead to unintended consequences, like the
inability to syntax highlight a unified diff. This resolves issues like
that by using .*? instead of .* so that the closing +++ is matched as
soon as possible.

src/front_matter.rs

@@ -11,7 +11,7 @@ use errors::{Result, ResultExt};
 
 
 lazy_static! {
-    static ref PAGE_RE: Regex = Regex::new(r"^\n?\+\+\+\n((?s).*(?-s))\+\+\+\n?((?s).*(?-s))$").unwrap();
+    static ref PAGE_RE: Regex = Regex::new(r"^\n?\+\+\+\n((?s).*?(?-s))\+\+\+\n?((?s).*(?-s))$").unwrap();
 }