feat: send custom command api

[robertsLando]

Fixes #336

[coveralls]

Pull Request Test Coverage Report for Build 518571432

• 19 of 78 (24.36%) changed or added relevant lines in 3 files are covered.
• 33 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.3%) to 21.584%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
types/index.d.ts	0	1	0.0%
lib/utils.js	19	30	63.33%
lib/ZwaveClient.js	0	47	0.0%

Files with Coverage Reduction	New Missed Lines	%
lib/utils.js	33	46.31%

Totals
Change from base Build 517679755:	-0.3%
Covered Lines:	2044
Relevant Lines:	9696

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💛 - Coveralls

[robertsLando]

Example using MQTT apis and calling startLevelChange command:

Topic: zwavejs/_CLIENTS/ZWAVE_GATEWAY-<yourName>/api/sendCommand/set

Payload:

{ "args": [
  {
    "nodeId": 23,
    "commandClass": 38,
    "commandClassName": "Multilevel Switch",
    "endpoint": 0,
    "property": "targetValue"
  }, 
  "startLevelChange",
  [{} ] // this are args, in this case it could be omitted
  ]
}