Add example flow to function node template

[kazuhitoyokoi]

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

I added functionality to create a sample flow for the function node template.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the mailing list/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage decreased (-0.04%) to 65.123% when pulling ad190e4 on kazuhitoyokoi:master-addexampleflow into 121e22e on node-red:master.