Add support of chained selectors

[btamas]

Add support for chained selectors.

From now the following css should work:

button {
    --width-small: 50px;
}

button.custom {
    --width-small: 100px;
}

button.rounded.small {
    width: var(--width-small);
}

button.custom.small {
    width: var(--width-small);
}

button.smallaswell.custom {
    width: var(--width-small);
}

Test: npm run test