Add exit code flag

[kakkoyun]

This PR adds a new parameter to API to ensure plugin fails for any type of error.
This is a hidden convenience flag to test the plugin in a container.

Fixes #58

Proposed Changes

• Adds --exit-code flag
• Fixes duplicate flag error introduced by feature: backend sftp #68 changes
• Adds a new pipeline step to the drone flow, to dogfood our own pipeline.

Description

• This PR adds a new parameter to API to ensure plugin fails for any type of error.
  This is a hidden convenience flag to test the plugin in a container.

Checklist

• Read the CONTRIBUTING document.
• Read the CODE OF CONDUCT document.
• Add tests to cover changes.
• Ensure your code follows the code style of this project.
• Ensure CI and all other PR checks are green OR
  • Code compiles correctly.
  • Created tests which fail without the change (if possible).
  • All new and existing tests passed.
• Add your changes to Unreleased section of CHANGELOG.
• Improve and update the README (if necessary).
• Ensure documentation is up-to-date. The same file will be updated in plugin index when your PR is accepted, so it will be available for end-users at http://plugins.drone.io.

[kakkoyun]

For the reviewer: This minor issue has introduced by the previous PR.

[kakkoyun]

For the reviewer: These newly added steps ensure dev tagged container up-to-date. That particular container is used to test the plugin on its on CI pipeline.

[kakkoyun]

@limayekaiwalya Have you had a chance to check this one?

[limayekaiwalya]

Not Yet @kakkoyun, I'll take a look tomorrow if that's okay.

[kakkoyun]

Hey @limayekaiwalya, I have a huge PR coming up, in order to prevent any merge conflicts, I'm going to go ahead and merge this one if you find any issues, please still comment and I'll address them in another PR.