Add a new option to support declarations aliases

[elchininet]

This pull request solves #65

This property consists of a string map to treat some declarations as others, very useful to flip the values of CSS variables

input

:root {
    --my-padding: 1rem 1rem 1.5rem 1.5rem;
}

.test {
    padding: var(--my-padding);
}

No aliases string map (default)

output

:root {
    --my-padding: 1rem 1rem 1.5rem 1.5rem;
}

.test {
    padding: var(--my-padding);
}

Set an aliases string map

const options = {
    aliases: {
        '--my-padding': 'padding'
    }
};

output

[dir="ltr"]:root {
    --my-padding: 1rem 1rem 1.5rem 1.5rem;
}

[dir="rtl"]:root {
    --my-padding: 1rem 1.5rem 1.5rem 1rem;
}

.test {
    padding: var(--my-padding);
}