Add dev flag to poetry installer

[Yanpas]

For #5496

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Has a news entry file (remember to thank yourself!)
• Appropriate comments and documentation strings in the code
• Has sufficient logging.
• Has telemetry for enhancements.
• Unit tests & system/integration tests are added/updated
• Test plan is updated as appropriate
• package-lock.json has been regenerated by running npm install (if dependencies have changed)
• The wiki is updated with any design decisions/details.

By default new dependencies like yapf, autopep8 are installed to all dependencies, --dev installs them to tool.poetry.dev-dependencies

[msftclas]

All CLA requirements met.

[ethanhs]

Just ran into this today. Would love to see it merged so it will make into 2019.5. Thank you for an awesome extension!