Particle.publish() flags fixes

[sergeuz]

This PR sanitizes Particle.publish() overloads, making it possible to specify one or two flags for any variant of this method. The PR also implements a generic type-safe wrapper for enum-based flags, aka particle::Flags<T>, which enables regular syntax for ORed flag combinations as an option:

Particle.publish("event", "data", PRIVATE | NO_ACK);

Fixes #1201 and #1194.

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Doneness:

• Contributor has signed CLA
• Problem and Solution clearly stated
• Code peer reviewed
• API tests compiled
• Run unit/integration/application tests on device
• Add documentation
• Add to CHANGELOG.md after merging (add links to docs and issues)

Bug fixes

• [PR #1236] [Fixes #1201] [Fixes #1194] Sanitized Particle.publish() overloads.

Enhancements

• [PR #1236] [Fixes #1201] [Fixes #1194] Added type-safe wrapper for enum-based flags for Particle.publish() which enables logical OR'ed flag combinations PRIVATE | WITH_ACK