Cannot include literal `&` with the nested plugin

__Expected behavior:__
A clear and concise description of what you expected to happen.

A way to escape `&` so the `nested` plugin does not auto-convert it to the selector.

__Describe the bug:__
_A clear and concise description of what happend instead of the expected behavior._

The following CSS cannot seem to be produced in JSS (with the nested plugin enabled):

```
someClass: {
'&[some-attr="A & B"]': { }
}
```

It gets turned into:

```
.someClass[some-attr="A .someClass B"] {}
```

__Codesandbox link:__
Please create a codesandbox.io with the issue. Make it as minimal as possible as this will help us find the bug quicker.

__Versions (please complete the following information):__
- jss: 10 alpha 18
- Browser [e.g. chrome, safari]: Chrome
- OS [e.g. Windows, macOS]: OS X

_Feel free to add any additional versions which you may think are relevant to the bug._

The regex in question [looks to be here](https://github.com/cssinjs/jss/blob/master/packages/jss-plugin-nested/src/index.js):
`const parentRegExp = /&/g`

A possible fix might be escaping with `&&` and changing the regex to `/(?:^|[^&])&(?:[^&]|$)/g`. There may be a more performant option (if that is a concern here).

Start in packages/jss-plugin-nested/src/index.js, especially the parentRegExp handling of ampersands, and reproduce the issue with the nested style shown in the report. Verify that the generated selector preserves a literal ampersand inside the attribute value while still expanding the nesting ampersand.

Written by the indexing model from the issue text.