Add ability to change merge method

* Add input option to action.yml, defining new possible  values and
  default value
* Read new input in JS and pass desired merge method to Oktokit
* Update documentation to highlight new input option

Re gr2m#25

README.md

@@ -23,6 +23,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: gr2m/merge-schedule-action@v1.x
+        with:
+          # Merge method to use. Possible values are merge, squash or
+          # rebase. Default is merge.
+          merge_method: squash
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 ```

action.yml

@@ -6,3 +6,10 @@ branding:
 runs:
   using: "node12"
   main: "dist/index.js"
+inputs:
+  merge_method:
+    description: |-
+      Merge method to use. Possible values are `merge`, `squash` or
+      `rebase`. Default is `merge`.
+    required: false
+    default: merge

lib/handle_schedule.js

@@ -12,6 +12,8 @@ async function handleSchedule() {
 
   const eventPayload = require(process.env.GITHUB_EVENT_PATH);
 
+  const mergeMethod = process.env.INPUT_MERGE_METHOD
+
   core.info(`Loading open pull request`);
   const pullRequests = await octokit.paginate(
     "GET /repos/:owner/:repo/pulls",
@@ -56,6 +58,7 @@ async function handleSchedule() {
       owner,
       repo,
       pull_number: pullRequest.number,
+      merge_method: mergeMethod
     });
 
     // find check runs by the Merge schedule action