Make sure source gets passed in as a string

I noticed that if you passed in just a number, JS
would helpfully parse that and then
`source.length` was undefined, which broke the
lexer.

bin/oslo.js

   }
 
   inline(source) {
-    return oslo(source, this.context)
+    return oslo(source.toString(), this.context)
   }
 
   absolutePath(file, root = null) {