Add bin/release script

Adds a simple bin/release script that helps automate the bumping of a
version and pushing a tag as well as running `mix hex.publish`.

Usage: `bin/release previous current`

Example:

```
bin/release 2.4.0 2.5.0
```

bin/release

@@ -0,0 +1,14 @@
+#!/usr/bin/env bash
+
+set -e
+
+previous_version="${1}"
+release_version="${2}"
+
+sed -i '' "s/$previous_version/$release_version/" README.md
+sed -i '' "s/$previous_version/$release_version/" mix.exs
+
+git commit -m "Release version $release_version"
+git tag "v$release_version"
+git push origin "v$release_version"
+mix hex.publish