Does not recognize root element in some cases

It looks like this plugin [tries](https://github.com/MadLittleMods/postcss-increase-specificity/blob/f952606dca9ec9bd76ebc27065417e0c8c5c2824/index.js#L16) not to break `html` selectors, but it doesn't handle more complex selectors containing `html`.

Test case:

**Input:**

```css
html[data-whatintent=keyboard] .button:focus {}
```

**Actual output:**

```css
:not(#\9):not(#\9):not(#\9) html[data-whatintent=keyboard] .button:focus {}
```

These styles never end up being applied because `html` has no parent element

**Expected output:**

```css
html[data-whatintent=keyboard]:not(#\9):not(#\9):not(#\9) .button:focus {}
```

It should append stackableRoot to the root selector, instead of prepending

Start in index.js around line 16, where root selectors are handled, and reproduce the issue with the provided html[data-whatintent=keyboard] selector. Confirm that the root element remains valid and that the output matches the expected selector, with stackableRoot appended rather than prepended.

Written by the indexing model from the issue text.