Add "wait" target

[kevinburkesegment]

I've often found it useful to have a command that "waits" for a build to complete and then notifies me about it. When you invoke it it will:

• Find the git commit and branch for the repo for the current working directory
• Locate a build in Buildkite for that commit (or wait for one to be created)
• Poll until the build to complete (you can use the most recent build time for guidance on how often to poll)
• If it completes, print out summary statistics of how long each step took
• if it fails, open a browser tab to the build/build step with information about the failed build.

You can see an example here: https://github.com/kevinburke/go-circle#usage

Here's some of the "wait" logic: https://github.com/kevinburke/travis/blob/master/main.go#L217-L341