Overview:
Now that we've added doctl support for uptime checks (#1386), we should follow up with supporting uptime alerts so that we support the entire feature set.

Instructions

1. Clone this repo
2. Create file: doctl/commands/uptime_alerts.go
3. Reference this PR and the API documentation for creating support for uptime alerts in doctl. You will need to use the digitalocean/godo package to make the API calls. Refer to uptime checks: add crud methods #1386 on how that was done.
4. Create a test file.
5. Create PR for us to review and merge 🎆