Add a menu checkbox to switch to canary/stable update channel

We really need a better way to change Hyper update channel.

The idea is to add an item Canary Releases right above Preferences... menu item in:

• Hyper menu on macOS
• About menu on Windows/Linux

This checkbox should be synchronized with config file:

• Checking it will update/add an "updateChannel" setting with a "canary" value.
• Unchecking it will set "updateChannel" setting to "stable" value.

If config file is updated manually, checkbox should reflect the new value. If the value is not 'canary' (or missing), it should be considered as 'stable'.

All other part of config file should remain unmodified.
To achieve this, using recast could be a great option. Example CLI that modifying plugins array in config file:

• Parse file content: https://github.com/zeit/hyper/blob/76beaa32f5f3285bc8f7083fff8cd722ba97f60a/cli/api.js#L38
• Finding the right setting: https://github.com/zeit/hyper/blob/76beaa32f5f3285bc8f7083fff8cd722ba97f60a/cli/api.js#L42
• Modifying the value: https://github.com/zeit/hyper/blob/76beaa32f5f3285bc8f7083fff8cd722ba97f60a/cli/api.js#L84
• Writing modified file: https://github.com/zeit/hyper/blob/76beaa32f5f3285bc8f7083fff8cd722ba97f60a/cli/api.js#L61

Trace the Hyper menu on macOS and the About menu on Windows/Linux, then inspect the config-editing flow in cli/api.js, especially parsing, setting lookup, modification, and writing. Confirm how the checkbox reads and updates updateChannel while preserving other config content; done means both menu locations stay synchronized with canary or stable values.