TV shows don't show up if new episodes come out after a series is watched entirely

*I'm submitting a ...*
- [X] bug report
- [X] feature request
- [ ] support request

## General infomration
If you have watched all episodes of a tv show , the show doesn't show up anymore if you set the list to 'unwatched', this is correct. However, if a new season op episodes comes out, the series is still marked as watched and will not appear in any list (my list, new releases, search etc). You have to manually set the plugin to show watched video's.

### Prerequisites

* [x] Are you running the latest version?

### Description
If you have watched all episodes of a tv show , the show doesn't show up anymore if you set the list to 'unwatched', this is correct. However, if a new season op episodes comes out, the series is still marked as watched and will not appear in any list (my list, new releases, search etc). You have to manually set the plugin to show watched video's.
Even removing and reinstalling the plugin in it's entirety does not solve the problem.

### Steps to Reproduce

1. Watch all episodes of a tv show (or mark as watched)
2. Wait until a new season appears on Netflix

**Expected behavior:** [What you expected to happen]
3. The show should re-appear in the menu's as unwatched, preferably with a 'NEW EPISIODES" marker.

**Actual behavior:** [What actually happened]
3. The show will not re-appear anywhere, it won't even show up in the search

## Context (Environment)
Liberelec nightly

### Installation

* [ ] I installed the plugin via zip from the Releases page
* [X] I´m using the Netflix Repo
* [ ] I´m using a different source (Please tell which)

### Operating System

* [X] Linux (x86/x64)
* [ ] OSX (x86/x64)
* [ ] Windows (x86/x64)
* [ ] Linux (ARM)
* [ ] Android

#### Additional informatin on the environment
Libreelec Millhouse 20180312

## Debug log
N/A

## Other information
A possible awesome addition would be a way to get notified of new released episodes. Especially because the series seems to be forever marked as watched, there is no way to tell if there are new episodes of a series other then to go into every season of every show and look at the checkmarks.

### Screenshots

No file or test is identified. Start by reproducing the watched-series flow with a fully watched show and a newly available episode, then trace the plugin's watched-state, menu, and search entry points. Done means the show reappears as unwatched after a new episode or season is available, without enabling watched videos.

Written by the indexing model from the issue text.