feat(translations): add menuitems translations

[asierrayk]

Prerequisites

• My Pull Request is filled against the next branch
• All commits in my Pull Request conform to Elegant Git commit Guidelines

Recommended Steps

• My patch adds a new feature, therefore I have also added a help article about it
• My patch changes Elegant behavior, therefore I have updated the help article to reflect this change
• My commits are signed

Description

Now it's possible to define custom values for menuitems. Home is separated from the rest since you have the ability to add pages as menu item between them.

HOME_MENUITEM = 'Inicio'

MENUITEMS = (
    ('categories', 'Categorías', CATEGORIES_URL),
    ('tags', 'Etiquetas', TAGS_URL),
    ('archives', 'Archivos', ARCHIVES_URL),
)

first value in tuple is needed to enable active class of the menu item

Close #139