Add step output "version" parameter for other parts of the workflow (#24)

* Add output.version

* Handle case where event has no commits (pull_request close)

Author: benwinding

merge-release-run.js

@@ -40,7 +40,7 @@ const run = async () => {
     }
   }
   if (!latest) {
-    messages = event.commits.map(commit => commit.message + '\n' + commit.body)
+    messages = (event.commits || []).map(commit => commit.message + '\n' + commit.body)
   }
 
   let version = 'patch'
@@ -60,6 +60,8 @@ const run = async () => {
   exec(`npm publish`)
   exec(`git checkout package.json`) // cleanup
   exec(`git tag ${newVersion}`)
+  exec(`echo "::set-output name=version::${newVersion}"`) // set action event.{STEP_ID}.output.version
+  
   /*
   const env = process.env
   const remote = `https://${env.GITHUB_ACTOR}:${env.GITHUB_TOKEN}@github.com/${env.GITHUB_REPOSITORY}.git`