Support for custom aggregate functions

[groue]

As requested by @hashemi in #236, here is the support for custom aggregate functions:

struct MySum : DatabaseAggregate {
    var sum: Int = 0
    
    mutating func step(_ dbValues: [DatabaseValue]) {
        if let int = Int.fromDatabaseValue(dbValues[0]) {
            sum += int
        }
    }
    
    func finalize() -> DatabaseValueConvertible? {
        return sum
    }
}

let dbQueue = DatabaseQueue()
let fn = DatabaseFunction("mysum", argumentCount: 1, aggregate: MySum.self)
dbQueue.add(function: fn)
try dbQueue.inDatabase { db in
    try db.execute("CREATE TABLE test(i)")
    try db.execute("INSERT INTO test(i) VALUES (1)")
    try db.execute("INSERT INTO test(i) VALUES (2)")
    try Int.fetchOne(db, "SELECT mysum(i) FROM test")! // 3
}

[groue]

Shipped in v1.1