Replaced multi-field type with multi-field syntax.

Replaced the multi-field type in favour for the multi fields option that can be set on any core field. The `path` option isn't changed in this PR.

Options defined in the main core field, are treated as defaults for any multi fields defined under this core field.
When upgrading to ES 1.0 the existing mappings with a multi-field type automatically get replaced to a core field with the new `fields` option.

Note: Any multi-field type defined in a dynamic template or index template will not be automatically replaced by a core field with multi fields. When dynamic templates or index templates are trigger and multi-field type is defined, then this will be converted in a core field with multi fields in the target mapping being generated.

Relates to #4521