Ensure opts.config exists before checking it

Without this I get the following:
```
TypeError: Cannot use 'in' operator to search for '../../../../common/assets/js/flashMessage' in undefined (while literalify was processing /.../assets/js-browserify/app/flashMessageTemplate.coffee)
    at /.../common/node_modules/literalify/index.js:5:22
    at /.../common/node_modules/literalify/node_modules/browserify-transform-tools/lib/transformTools.js:241:16
    ...
```
I'm also using coffeeify and browserify-middleware - and using package.json to specify literalify options.