Fix BufferSettings append & add ParseFrontMatter

until now buffersettings were always appended using \n which means the first
value would already be written as "\nVALUE". Not anymore.

Also we finally add an option to parse just the front matter. Still not
efficient as we tokenize the whole org file but i don't think saving a few
milliseconds would be worth making the code uglier.

org/keyword.go

@@ -47,7 +47,11 @@ func (d *Document) parseKeyword(i int, stop stopFn) (int, Node) {
 			return consumed, node
 		}
 	}
-	d.BufferSettings[k.Key] = strings.Join([]string{d.BufferSettings[k.Key], k.Value}, "\n")
+	if _, ok := d.BufferSettings[k.Key]; ok {
+		d.BufferSettings[k.Key] = strings.Join([]string{d.BufferSettings[k.Key], k.Value}, "\n")
+	} else {
+		d.BufferSettings[k.Key] = k.Value
+	}
 	return 1, k
 }