win32: Rename the uninstall exe.

Resolves #467.
cli: Revert Twitch OAuth callback URL change.
docs: Improve docs about the config file slightly.
Add LetOn.tv plugin
plugins.letontv: Make use of the validation API.

Also add support for leton.tv/broadcast/<channel> URLs.
Update AUTHORS.
Merge pull request #500 from cheah/leton.tv-plugin

Add LetOn.tv plugin
Update CHANGELOG.rst.