Enable custom command to have arguments in keybindings

[gowerc]

It would be really useful to adjust custom command to accept arguments so that we can bind specific actions to keyboard shortcuts i.e.

    "key" : "shift+alt+l",
        "command": "code-runner.runCustomCommand",
        "args" : { "command": "echo  some_command"}
    },

    "key" : "shift+alt+m",
        "command": "code-runner.runCustomCommand",
        "args" : { "command": "echo  some_other_command"}
    },

    "key" : "shift+alt+e",
        "command": "code-runner.runCustomCommand",
        "args" : { "command": "echo  yet_some_other_command"}
    }

[Phrogz]

See also Issue #142, Issue #380, Issue #553, Issue #608