Add structure for JavaScript assets

### What we have

In the current boilerplate, one single JS file is loaded on all the pages (`public/wp-content/themes/coloredcow/main.js`). This file is compiled from `public/wp-content/themes/coloredcow/src/js/main.js`.

### Issue

With a single JS file, we missed out on the opportunity. There can be page-specific JavaScript like slider may be needed on a single page. I think it makes sense to load it on the pages on which we need it.

### Possible solution 1

1. Have a single JS that contains generic code: `public/wp-content/themes/coloredcow/main.js`
1. Have multiple page-specific JS files: `page-about.js`, `page-team`
1. Running grunt will combine `main.js` with each page's JS file will create minified files for each file.
1. We will enqueue based on the page's slug

### Possible solution 2

1. The first two steps are the same as above.
1. Grunt will **not** combine the page-specific JS file with the main.js and generate the just the minified file.
1. There will be two files that will be enqueued on each page (hence two requests): `main.js` and `page-*.js`

@rathorevaibhav would love to know your thoughts/questions.