support "command" input variable type

[weinand]

Input variables have introduced an excellent way to parameterize and configure builtin interactive variables. But they lack support for contributing new variables.

But there is an easy way to combine the new input variables with the existing command based variables: a new input variable type "command" makes it possible to turn any command based variable into an input variable.

Example:

	"inputs":[
		{
			"label": "processPicker",
			"description": "Select process to attach to",
			"type": "command",
			"command": "extension.processes.pickProcess",
                         "args": {
			    "processTypes": [ "node", "iojs", "electron" ]
                         }
		}
	],

/cc @alexr00

[weinand]

• a user input of type command is now supported (and there are tests for it).
• variables are now prompted for in a single pass: as a result input and command variables are now evaluated in the order they appear in the task or launch config.
• cancelling a prompt aborts the task or launch config.
• validation of an input structure only occurs for the needed ones; so errors in other inputs are not flagged and don't confuse the user.
• intellisense now uses the "enumDescriptions" schema feature.