ContextSuggester
================

This commit extends the `CompletionSuggester` by context
informations. In example such a context informations can
be a simple string representing a category reducing the
suggestions in order to this category.

Three base implementations of these context informations
have been setup in this commit.

- a Category Context
- a Geo Context

All the mapping for these context informations are
specified within a context field in the completion
field that should use this kind of information.