Add Probot apps to improve workflow (#329)

* Add Probot Stale app

* Add Probot Welcome app

.github/config.yml

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+# Configuration for new-issue-welcome - https://github.com/behaviorbot/new-issue-welcome
+
+# Comment to be posted to on first time issues
+newIssueWelcomeComment: |
+  👋 Thanks for opening your first issue here! If you're reporting a 🐞 bug, please make sure you include steps to reproduce it. We get a lot of issues on this repo, so please be patient and we will get back to you as soon as we can.
+
+  To help make it easier for us to investigate your issue, please follow the [contributing instructions](https://github.com/whipper-team/whipper#bug-reports--feature-requests).
+
+# Configuration for new-pr-welcome - https://github.com/behaviorbot/new-pr-welcome
+
+# Comment to be posted to on PRs from first time contributors in your repository
+newPRWelcomeComment: >
+  💖 Thanks for opening your first pull request here! 💖
+
+# Configuration for first-pr-merge - https://github.com/behaviorbot/first-pr-merge
+
+# Comment to be posted to on pull requests merged by a first time user
+firstPRMergeComment: >
+  Congrats on merging your first pull request! 🎉🎉🎉

.github/stale.yml

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+# Configuration for probot-stale - https://github.com/probot/stale
+
+# Number of days of inactivity before an Issue or Pull Request becomes stale
+daysUntilStale: 30
+
+# Number of days of inactivity before an Issue or Pull Request with the stale label is closed.
+# Set to false to disable. If disabled, issues still need to be closed manually, but will remain marked as stale.
+daysUntilClose: 7
+
+# Issues or Pull Requests with these labels will never be considered stale. Set to `[]` to disable
+exemptLabels:
+  - Accepted
+
+# Set to true to ignore issues in a project (defaults to false)
+exemptProjects: false
+
+# Set to true to ignore issues in a milestone (defaults to false)
+exemptMilestones:
+  - "2.0"
+  - backlog
+
+# Label to use when marking as stale
+staleLabel: "Status: stale"
+
+# Comment to post when marking as stale. Set to `false` to disable
+markComment: >
+  This issue/pull request has been automatically marked as stale because it has not had
+  recent activity. It will be closed in 7 days if no further activity occurs. Thank you
+  for your contributions.
+
+# Comment to post when removing the stale label.
+unmarkComment: >
+  Thank you for updating this issue. It is no longer marked as stale.
+
+# Comment to post when closing a stale Issue or Pull Request.
+closeComment: |
+  This issue/pull request has been closed due to prolonged inactivity.
+
+  If you think this is an error, please leave a comment and we will gladly reopen it.
+
+# Limit the number of actions per hour, from 1-30. Default is 30
+limitPerRun: 30