Add language detection option

Based on PR #45, we add a new language detection option using Language detection feature available in Tika:
https://tika.apache.org/1.4/detection.html#Language_Detection

By default, language detection is disabled (`false`) as it could come with a cost.
This default value can be changed by setting the `index.mapping.attachment.detect_language` setting.
It can also be provided on a per document indexed using the `_detect_language` parameter.

Closes #45.
Closes #44.