Implement entity tag deletion

[ctrombley]

Objective

Support Entity Tag deletion via the CLI.

Acceptance Criteria

• Add an entities subcommand for deleting an entity's tags via entity guid
• Add an entities subcommand for deleting an entity's tag/value pairs via entity guid
• Provide help and example command invocation
• CLI error code should match success/failure of the operation
• The pflag StringSlice and StringArray types can be used to support repeated flag instances during a call to a CLI command

Notes

Potential command structure:
newrelic entities delete-tags --guid <guid> --tags tag1,tag2
`newrelic entities delete-tag-values --guid --value tag1:value1 --value tag2:value2