fix(module): fix "dependency not found ~\content" (#78)

Incorrect use of `path.join` in webpack context causing the use of "\"
on windows

Author: medfreeman

lib/module.js

@@ -125,7 +125,7 @@ export default function ContentModule (moduleOpts) {
   this.addPlugin({
     src: resolve(__dirname, 'plugins/markdownComponents.js'),
     options: {
-      srcDirFromPlugin: join('~/', srcDir)
+      srcDirFromPlugin: '~/' + srcDir
     }
   })
 }