Option to set a url

[Frederik Heyninck]

Option to set a url to a website

[alies-lptn]

@freshface
how many services support it? Your PRs are welcome

Note about package principles:

1. it should produce a small output (to keep pagesize small)
2. it should be fast (no any external heavy dependencies)
3. all features should be supported by at least 2 generators (different services have different features)