expansion_limit search option

[sanikolaev]

The task is to expose expansion_limit as a search option, so it's available in SELECT ... OPTION= and as a search option in the /search endpoint in the JSON API.

[tomatolog]

the feature should be added at 1a594c3

and now SphinxQL query supports SELECT ... OPTION expansion_limit=X and HTTP /search request supports

{ 
  "index": "test", 
  "query": { 
    "query_string": "*fu*" 
    } 
    , 
  "options": { 
    "expansion_limit": 5
  }
}

this overrides searchd.expansion_limit option

[tomatolog]

documented option at 8c466c5