A logic to generate an empty plugin placeholder.

[mashhurs]

What does this PR?

• Adds a logic to generate an empty placeholder for new plugin
  • currently we have an use-case for integration plugin only;
  • it uses common template which later on can be used to fill released notes and dates;
• This change also adds a git helper to DRY.
  • git operations are used in both VPR docs generator and placeholder generator, so created a git helper to DRY;

Test

• Generated sample: [DO NOT MERGE] create an empty placeholder for new plugin logstash-docs#1536
• How to run locally?
  • Pull this change and run ruby generate_plugin_placeholder.rb --output-path=/Users/mashhur/Dev/elastic --plugin-type=integration --plugin-name=testing command

Related issue

• Closes VPR: Adjust tooling to improve workflow for adding new integration plugin docs #90

[jsvd]

To avoid confusion with the plugin generator I suggest we scope the naming to include Docs

Suggested change

	class GeneratePluginPlaceholder < Clamp::Command
	class GeneratePluginDocsPlaceholder < Clamp::Command

[mashhurs]

WDTY for GeneratePluginPlaceholderDoc? Docs sounds the script generating non-single doc.

[jsvd]

LGTM