Split Listing into two types, one of which automatically marshals children

[EliteMasterEric]

This resolves a flawed assumption that all listings returned by the Reddit API output Model objects; many return generic objects, such as the [/r/subreddit]/api/flairlist endpoint.

Fixes issue #93.

[drice89]

@MasterEric is there a reason why your changes haven't been merged in yet? Is this repo still active?

[EliteMasterEric]

@drice89 It appears that the repository has not been committed to in about 2 and a half years.

I made this commit when they hadn't pushed in about a year, but now the repo is definitely unmaintained.

I don't fully recall this pull request, but judging on this and my other PR, the library is missing many key functions, and probably does not support any of the functionality that has modified since the release of the new interface.

I don't know of any alternatives for Reddit bots for Ruby. Sorry.

[drice89]

@MasterEric I was able to update the dependencies after forking it and I got a bot to post to a sub, so it does still work. I thought this commit was from feb 2020 not 2019 so my apologies - I didn't read that right. I was going to ask if you were interested in forking and upgrading this but since it was from a year ago, I am guessing you moved on.