Refactor to improve require syntax

My motivation behind this was to understand how this lib worked and if there was a way to make using it easier.

Old:

```javascript
dotenv = require('dotenv')();
dotenv.load();
```

New:

```javascript
require('dotenv');
```

I'm trying to understand the need for environment files like `.env.development`. If you're keeping `.env` out of version control, then you development values should be able to go directly into `.env` so there wouldn't be a need for `.env.development`. What are some example situations I'm overlooking?

I kept the ability to include these as you like:

```javascript
dotenv = require('dotenv');
dotenv.load('.env.development');
```