YouTube supports channels identified by both channel ID and channel
name. For example, https://www.youtube.com/c/dslrvideoshooter and
https://www.youtube.com/channel/UCMmA0XxraDP7ZVbv4eY3Omg are the same
channel, but the former has a more human-friendly URL. This change
allows the extension to detect the channel's RSS feed regardless of
which URL is loaded using the canonical link provided on each channel
page. This change is fully backward compatible.
Refactor the `findFeeds` function to allow easy addition of custom
finder functions. This change does three things:
1. Breaks the iTunes and standard feed detection logic into their own respective functions
2. Adds a YouTube feed detection function
3. Creates a mapping to support custom feed detection functions for
additional domains
Fixes#34.
Currently, if a feed provides summaries of posts, it's not possible to
view the full content within the preview page. This change adds a new
option to choose between summary and full contents.
