Add exists query

[hipsterjazzbo]

https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-exists-query.html

[sleimanx2]

Yeah seems useful, but how about we create an array argument option so we get a cleaner syntax in case of multiple field existence check ?, ex:

instead of

News::search()->exists('body')->exists('tags')->get();

we get

News::search()->exists(['body','tags'])->get();

... like the sortBy method

[hipsterjazzbo]

Good idea. Doneskis. Great package BTW 👍