broken attribute selectors with `\a` and ` ` characters

[alexander-akait]

Unfortunately this example fails at the moment. It turns into \a into a (removes only the \). It seems a different problem though.

Originally posted by @ludofischer in https://github.com/postcss/postcss-selector-parser/pull/245#r629639304

Example:

// http://jsfiddle.net/BoltClock/AHuvh/6
a[href="\a  http://google.com"] {
  color: red;
}

[alexander-akait]

Let's fix it late, anyway PR welcome