Suggestion: add channel id, channel descprition and icon_url in config.yaml

[wangfeng35]

Fisrt, Thanks for your great work!
I used this project to convert youtube channel into podcast so I can listen to it. However, there're some problems.

1. The name in config.yaml is used to generated channel_title , channel_link, icon_url item.url and item.thumbnail in rss file. Is it possible to add channel_id for generating the url link except channel_title, so that the name can use non-English characters (for example Chinese) in channel_title. If the channel_id is not configured, it will equals to name.
2. The generated rss file does not have any description of the channel. Maybe it's better to configure channel_descprition in the config.yaml?
3. For some channels that don't have icon_url, is it possible to configure the url directly in the config.yaml?

The new template may be changed to this:

<channel>
    <updated>{{ last_update }}</updated>
    {% if channel_id %}<id>{{ channel_id }}</id>{% endif %}
    <title>{{ channel_title }}</title>
    {% if channel_description %}<description>{{ channel_description }}</description>{% endif %}
    <link href="{{ channel_link }}" rel="self" type="application/rss+xml"/>
    {% if icon_url %}
    <itunes:image href="{{ icon_url }}"/>
    {% endif %}

The config.yaml may be changed to this:

subscriptions:
  - channel_id: 1
    name: 播客1
    url: https:youtube.com
    channel_description: 这是一个示例
    icon_url: https//:youtube.com.png
    retention_days: 14 # How old is the oldest video to download and keep locally
    initialize: False # Download the whole channel on first run
    download_last: 5 # Only download the last video at each run
    audio_only: True # Extract and keep audio only
    format: m4a      # Force end format
    filename_template: '%(id)s.%(ext)s' # Use this format for files saved from this