Add command to create releases.

This PR adds a new command to create releases with gh.

It combines two steps in a single command.

Running `gh release v1.0.0` this command will perform two tasks:

1. Create a release in your github repo for the tag v1.0.0.
2. Upload any asset that it finds under the directory `releases/v1.0.0`.

The location of the assets can be modified using the flag `-a`.
The flags `-p` and `-d` can be passed to create a prerelease or a draft.

This command will open EDITOR if there is no message or file to get the title and the body from.