Added support for single quotes in css.

[tralves]

Fixes #20.

Currently, the plugin doesn't support single quotes on CSS. This means that this doesn't work:

.fa-heart:before {
    content: '\f004';
}

This PR changes the cleanValue regex to support both " and '.

I added/changed tests and changed the demo app, so I know it works.

Thanks!

PS: The real reason why I made this PR is because I couldn't get prettier/eslint/vscode to stop converting " to ' in my CSS file 😎...

[brendtumi]

Hi @NathanWalker , any reason for not merging this PR ?

[devlinpadayachee]

looks like he has given up on this lib

[NathanWalker]

@devlinpadayachee hi - if this fixes a current issue for you just ping back and perhaps we can bring this lib under scope so it’s more visible and tended to.
Cc @brendtumi @tralves