Draft of a subtitle queue

I'm tired of having a separate program downloading subtitles (or scanning my files recursively), so I have created a very rough draft of a subtitle queue, which a subtitle plugin such as Subliminal can use as input.

This is the schema (`emit` is gonna be moved out of the dict, hence the `oneOf`):
```python
schema = {
        "oneOf": [
            {
                'type': 'object',
                'properties': {
                    'action': {'type': 'string', 'enum': ['add', 'remove', 'emit']},
                    'stop_after': {'type': 'string', 'format': 'interval'},
                    'languages': {"oneOf": [
                        {'type': 'array', 'items': {'type': 'string'}, 'minItems': 1},
                        {'type': 'string'},
                    ]},
                },
                'required': ['action'],
                'additionalProperties': False
            },
        ]
    }
```
Example tasks:
```yaml
populate_subtitle_queue:  # just run this once per session to keep queue uptodate
    seen: local
    find:
        path:
          - /media/TV Shows/
        regexp: '.*\.(avi|mkv|mp4)$'
    accept_all: yes
    regexp:
        reject:
          - '.*[sS]ample.*'
    subtitle_queue:
        action: add

torrents:
   discover:
        what:
            - emit_series:
                from_start: yes
        from:
            - piratebay: yes
        series:
           - "Arrow":
              begin: S02E01
   subtitle_queue:
        action: add
        languages:
           - eng
   set:
        path: /media/downloads
        movedone: "/media/TV Shows/{{series_name}}"
        content_filename: "{{ series_name }}"
   magnets: no

emit:
   subtitle_queue:
        action: emit
   subliminal:
        exact_match: yes
```
So what does this ugly code do?

The plugin has the ability to queue local files or torrents (does not support magnets). Add the contents of a torrent to the subtitle queue -- if torrent only contains 1 file, then that file is queued and if it contains more than 1 file, then the folder is queued instead.

What annoyed me most about something like this is that sorting tasks will mess with the queue. Therefore this plugin can be used in the same task as `move` and it will add the destination as an alternative filepath in the queue (same goes for torrents wrt. `path` and `movedone`).

`stop_after` is an interval that will drop queued items after the specified interval.

This is all still very rough around the edges and I'm not even sure if it works just yet, but it's at a stage where some third party input would be much appreciated.
Well, I don't think we should be messing around with torrent files in this plugin at all, especially not downloading them again as this seems to be doing right now. Looks like the stuff for content_files is incomplete at the moment though. We should split this bit out from content_filter plugin, and be filling the entry field from some other plugin https://github.com/Flexget/Flexget/blob/d53e59b75e81abff0f794b723e1c9df76f6d8f39/flexget/plugins/filter/content_filter.py#L94-L99, similar to the way content_size works. This way we don't need to care what kind of entry we are working on from this plugin, just check that field.