addd support to internatinalization

[zerasul]

add support to internationalization and choose the same page using a metadata.

[JuanjoSalvador]

Hmmm... Something like this?

mydomain.com/es/homepage -> spanish version
mydomain.com/en/homepage -> english version

But having a metatag specifying the language?

[zerasul]

in this case we have two approachings:

1. Read the http accepted_languages header; then, search for the file homepage-< lang >.md. If it's not found, search the default .md file. Example -> accepted_languages=['es'] -> homepage-es.md
2. put in the URL the -lang suffix. If there is no this file, search for the default md file.