Resolves asciidoctor#35, automatically register converter and extension

lib/cli.js

@@ -33,7 +33,14 @@ function convertOptions (argv) {
     console.log('destination-dir ' + destinationDir)
   }
   if (requireLib) {
-    require(requireLib)
+    const lib = require(requireLib)
+    if (lib && typeof lib.register === 'function') {
+      // REMIND: it could be an extension or a converter.
+      // the register function on a converter does not take any argument
+      // but the register function on an extension expects one argument (the extension registry)
+      // Until we revisit the API for extension and converter, we pass the registry as the first argument
+      lib.register(asciidoctor.Extensions)
+    }
   }
   const verboseMode = quiet ? 0 : verbose ? 2 : 1
   const attributes = []