Fix possible error if HackMD is started with wrong workdir

In https://github.com/hackmdio/hackmd/issues/834 is described how
starting HackMD crashes when using the wrong working dir.

This is caused by a relative path in our upload routine. This change
should fix it and prevent future crashes.

Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>

lib/config/default.js

@@ -45,6 +45,7 @@ module.exports = {
   errorPath: './public/views/error.ejs',
   prettyPath: './public/views/pretty.ejs',
   slidePath: './public/views/slide.ejs',
+  uploadsPath: './public/uploads',
   // session
   sessionName: 'connect.sid',
   sessionSecret: 'secret',