Add typescript support

[BobCashStory]

[syffs]

@boblauer Your last commit on master was in Jan 2019, and this is the 3th opened PR (see also #60 #63 ) about typescript defs

Hence, could you please either review and merge those or give maintainer to whoever wants to do it ?

cheers

[syffs]

@BobCashStory FYI, here are latest typescript defs required with mongoose>5.11 which now includes its own defs (@types/mongoose is no longer needed):

declare module 'mongoose' {
    interface Query<ResultType, DocType extends Document>  {
        // cache(ttl?: number, customKey?: string): DocumentQuery<DocType | null, DocType> & QueryHelpers
        cache(ttl?: number, customKey?: string): this
    }
    interface Aggregate<R> {
        cache(ttl?: number, customKey?: string): this
    }
}