Cache translation from channel and user name to ID (translation prone to failure)

When a channel name is given for a channel parameter, rather than a channel ID, we are using channels.list to look up the ID of the channel and then using the ID in the request instead of the name.

However, channels.list is a legacy method and is no longer guaranteed to return every channel. This leads to a channel_not_found error if you pass a channel name for a channel that is not supported by channels.list.

A couple suggested solutions:

1. Update channels_id to use conversations.list instead of channels.list. The tricky thing about this is that conversations.list only works with pagination, so it's not a simple drop-in replacement. It also means that if you do something like chat_postMessage(channel: 'general', text: 'foo') then for a team with many channels you might end up making hundreds of paginated calls to conversations.list before actually hitting chat.postMessage, which is not ideal. If we use this solution I think we should change the readme to discourage this approach and encourage using channel IDs directly.
2. Remove the translation completely, which would be a breaking change (but not the worst breaking change, since it's already basically broken). Since using conversations.list is not ideal for making the translation (see above) we could remove support for this altogether, forcing people to confront the issue directly.
3. Leave it as it is, and update the readme to note that it does not always work and discourage its use.

See #270 for more background.

Start with lib/slack/web/api/mixins/channels.id.rb and the background in #270. Compare the three proposed approaches, including conversations.list pagination and the current README guidance, then establish which behavior should be supported. Done means the chosen channel-name translation policy is implemented and the README accurately documents it.