autocomplete for scripts at the top level

If scripts are found in a `package.json` file in the current directory,
add them as first-level autocompletions commands.

Example:

package.json
```json
{
    "scripts": {
	"foo": "echo bar"
    }
}
```

`yarn f<tab>` will complete to `yarn foo`

package.json

@@ -1,6 +1,6 @@
 {
   "name": "yarn-completion",
-  "version": "0.1.2",
+  "version": "0.2.0",
   "repository": "https://github.com/dsifford/yarn-completion.git",
   "description": "shell autocompletion for yarn",
   "author": "Derek P Sifford <[email protected]>",

yarn-completion.bash

@@ -1,6 +1,6 @@
 # shellcheck shell=bash
 #
-# Version: 0.1.2
+# Version: 0.2.0
 #
 # bash completion for Yarn (https://github.com/yarnpkg/yarn)
 #
@@ -469,6 +469,7 @@ _yarn() {
         upgrade-interactive
         version
         why
+        $( __yarn_get_package_fields scripts )
     )
 
     local global_flags=(