Method supportedValue should support !important property.

```js
it( 'should support !important property (solid 1px red)', () => {
expect( supportedValue( 'border', 'solid 1px red !important' ) ).to.be( 'solid 1px red !important' );
} );

it( 'should support !important property (1px solid red)', () => {
expect( supportedValue( 'border', '1px solid red !important' ) ).to.be( '1px solid red !important' );
} );
```

I added these two test cases for `supportedValue` method, and got the output as follow:

```
✖ should support !important property (solid 1px red)
✔ should support !important property (1px solid red)
```

```
✖ should support !important property (solid 1px red)
Chrome 86.0.4229.3 (Mac OS 10.15.6)
Error: expected false to equal 'solid 1px red !important'
at Assertion.assert (webpack://cssvendor/node_modules/expect.js/index.js:96:13 <- tests.webpack.js:8:10305)
at Assertion.be.Assertion.equal (webpack://cssvendor/node_modules/expect.js/index.js:216:10 <- tests.webpack.js:8:11808)
at Assertion. [as be] (webpack://cssvendor/node_modules/expect.js/index.js:69:24 <- tests.webpack.js:8:9695)
at Context. (webpack://cssvendor/src/supported-value.test.js:83:81 <- tests.webpack.js:29:255519)
```

This issue makes using `1px solid red !important` and `solid 1px red !important` has different result in `jss`. To support `!important` property or at least to provide same result with these two type of format.

I can send a pull request for this if you think it makes sense.