[Feature] alternate hreflang

[jwundrak]

Related to #17

Here is a first implementation of hreflang in Sitemap like documented here: https://support.google.com/webmasters/answer/2620865?hl=en

Because I don't use tt_news, I only implemented for pages (and my custom extension records).
The implementation works for me in different modes. Feel free, to comment. I don't know, if the implementation can more elegenat.

Notice: I introduced a new abstract class AbstractExtendedSitemapRenderer to keep backwards-compatibility for other record types.

[TomasJerrySebo]

This is a nice feature and it would be more than welcomed, will it merged?

[jwundrak]

I wrote last week an email to @dmitryd , but he is currently very busy and has no time, to look at this request:

Yes, the extension is still supported but I do not plan updates soon. I am quite busy at my daily job and nobody from my paying clients requested this functionality. Currently I have no time for work on my free extensions. So I cannot tell when this will be integrated.

The best thing, to complete the pull request by the community, if more people want's this feature. Then it should be more easier for dmitry to review the request. So please give me feedback and suggestions at my branch.

I commit some performance optimatimations, but tt_news implementation is still missing

[dmitryd]

The change is unnecessarily large and contains changes that are not related to the ticket in question. I may reuse some parts but I will not be able to use it as whole.