add support for nesting selector

[jantimon]

today ~76% of all browsers support for css nesting:

https://caniuse.com/css-nesting
https://developer.mozilla.org/en-US/docs/Web/CSS/Nesting_selector

it would be great if postcss-modules-local-by-default could also add support for nesting.

Today the following valid css code

.foo { &:hover { a_value: some-value; } } 

will throw:

CssSyntaxError: postcss-modules-local-by-default: <css input>:1:8: Selector "&:hover" is not pure (pure selectors must contain at least one local class or id)

I added a possible solution in pr #64