Don't require the input to end with a newline

Technically, not ending a line with a newline is wrong, but this blunder
is so pervasive that we have no other choice but to live with it.

Fixes #180.

inline_test.go

@@ -695,6 +695,9 @@ func TestReferenceLink(t *testing.T) {
 
 		"[ref]\n   [ref]: ../url/ \"title\"\n",
 		"<p><a href=\"../url/\" title=\"title\">ref</a></p>\n",
+
+		"[link][ref]\n   [ref]: /url/",
+		"<p><a href=\"/url/\">link</a></p>\n",
 	}
 	doLinkTestsInline(t, tests)
 }

markdown.go

@@ -638,9 +638,6 @@ func scanLinkRef(p *parser, data []byte, i int) (linkOffset, linkEnd, titleOffse
 	for i < len(data) && data[i] != ' ' && data[i] != '\t' && data[i] != '\n' && data[i] != '\r' {
 		i++
 	}
-	if i == len(data) {
-		return
-	}
 	linkEnd = i
 	if data[linkOffset] == '<' && data[linkEnd-1] == '>' {
 		linkOffset++