Add feature to generate slug from another language

[h3nn3s]

We needed a possibility to generate the slugs of some specific languages from another translation.
So we evaluated a bunch of extensions and found out, no one offers such a feature.

Something like this TSconfig could configure the SlugModifier:

TCEMAIN.pages.slug.generatorOptions {
    languageMapping {
        1 {
            target = 3
            source = 1
        }
        2 {
            target = 4
            source = 0
        }
    }
}

sys_language_uid 1 = german
sys_language_uid 2 = english
sys_language_uid 3 = chinese
sys_language_uid 4 = russian

Chinese pages would get english paths.
Russian pages would get path segments from default/german language

We think, this feature could fit best for EXT:masi. What do you think?
We could provide a PR for this feature, if you are interessted