Add FTS snippet functionality

Start with the VirtualTable, Expression, and QueryType APIs used in the example, then trace how .match("cold") builds and selects expressions. Compare that behavior with SQLite FTS3/4 snippet and matchInfo queries. Done means callers can select these FTS results and receive SQLite’s matched snippet or match information rather than a rendered expression description.

SQLite when using the FTS3/4 extensions can support the concept of a snippet. Essentially the snippet is a subset of the indexed column, that has html bold tags surrounding the matched terms.

Source information here: https://www.sqlite.org/fts3.html#section_4_2

So using the example from the above link, a snippet query it might look something like:

SELECT snippet(text) FROM text WHERE text MATCH 'cold';

I have successfully used the VirtualTable implementation to retrieve matching records using the .match(string: String) function, but would like to be able to select the snippet as well.

My current implementation attempt is:

let db = try? Connection("table.database")
let indexTable = VirtualTable("fts_index")
let guidField = Expression<String>("object_guid")
let contentsField = Expression<String>("contents")
let snippet = Expression<String>("snippet(contents)")
let _ = try? db?.run(indexTable.create(.FTS4(guidField, contentsField)))

let matchQuery: QueryType =indexTable.select(snippet, guidField, contentsField).match("cold")
let results = try? db?.prepare(matchQuery)

if let concreteResults = results {
  let _ = concreteResults.flatMap( { 
    print("\($0[guidField]), \($0[snippet]), \($0[contentsField])")
  })
}

I get what I expect for both guidField and contentsField, yet the result of snippet is something puzzling.

3771714D-19AB-464B-80E2-6C5A798D8104, snippet(Expression<String>(template: "\"contents\"", bindings: [])), This is a cold record

I imagine I just need a custom expression, but I am not sure how to implement custom SQLite functions, or else I may just be missing it in the docs.

Thanks for any help, and I would be glad once I know how to implement this for both the snippet and matchInfo capability in SQLite.

Thanks, awesome library and really enjoying using it.