add support for xml property 'seriescode', which is returned for things of type 'rpgitem'

[pants97]

I submitted this PR in order to fix this issue: #57

when requesting a thing of type 'rpgitem' the thing's dto parser errors out with an unknown property error:

Error: Unknown property "seriescode" for BggThingDto at [Source ...]
    at .\node_modules\boardgamegeekclient\dist\cjs\dto\dtoparser\concrete\BggThingDtoParser.js:12:33

steps to reproduce:

import {BggClient} from 'boardgamegeekclient'
BggClient.Create()
    .thing
    .query({id: 174620})
    .then(console.info)
    .catch(console.error)

fix: by adding definitions to support that property the parser does not fail anymore and produces a result as expected;

notes:

• thing id 174620 is that of "Blades in the Dark" rpgitem, found at: https://boardgamegeek.com/rpgitem/174620/blades-in-the-dark
• also tested with other rpgitems, like 146382 or 43842
• also added a test file, however I had no experience with jest so I couldn't make it run, but it follows the same pattern as other tests so I expect it to run; I ask your indulgence on this, thanks!