Add NULL reply style to allow disabling replies for handshakes

[pcarranza]

This fixes #93

As proposed the UI is:

format:
  reply_styles:
    handshake: disabled

But we can also say "nil" and "null", be mindful of the quotes because "null" is a yaml value, therefore it doesn't get interpreted as a string.

[coveralls]

Pull Request Test Coverage Report for Build 312

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 64.672%

---

Files with Coverage Reduction	New Missed Lines	%
remote/agent/agent.go	4	34.64%

Totals
Change from base Build 308:	-0.2%
Covered Lines:	1232
Relevant Lines:	1905

---

💛 - Coveralls

[omame]

Bah. I forgot to point out that we need a test for this.
I'll open an issue.