Support multiple background(-image) values
Somewhat related to #6. Looking at the source I don't see yet that it supports multiple box shadows. Which is a must for my use cases.

We should be able to support the following:

```
background-image: linear-gradient(to top, rgba(67, 90, 111, 0.06), rgba(67, 90, 111, 0.024));
box-shadow: rgba(67, 90, 111, 0.255) 0px 0px 0px 1px inset, rgba(67, 90, 111, 0.114) 0px -1px 1px 0px inset;
```

### General multi parser

It might be nice to have a function or parser that simply splits the parts. I wrote something similar for `background.js`. I am not great at algorithms, there might be better approaches to this thing :)

```js
const parts = [];
let currentPart = [];
let i = 0;
let skipComma = false;

// There can be commas in colors, carefully break apart the value
while (i < value.length) {
  const char = value.substr(i, 1);

  if (char === '(') {
    skipComma = true;
  } else if (char === ')') {
    skipComma = false;
  }

  if (char === ',' && !skipComma) {
    parts.push(currentPart.join('').trim());
    currentPart = [];
  } else {
    currentPart.push(char);
  }

  if (i === value.length - 1) {
    parts.push(currentPart.join('').trim());
  }
  i++;
}
```
