Lunr

- Fix Python 2.7 support

- Added multilanguage support
- Improved language support

Deprecation warning

- The `language` argument to the `lunr` has been renamed to `languages` to accomodate for multilanguage support. The `languages` argument accepts a string or an iterable of ISO-639-1 languages codes. If you're calling `lunr` with keyword arguments please update such calls accordingly.

- Target Lunr.js v2.3.2

- Target Lunr.js v2.3.1
- Fix crash when using non-string document references.

- Added support for Python 3.7

- Compatibility with Lunr.js v2.3.0. Including:
+ Add support for build time field and document boosts.
+ Add support for indexing nested document fields using field extractors.
+ Prevent usage of problematic characters in field names