Add optional key-values that will be outputted to JSON in config

[kotakanbe]

Optional key-value will be outputted to JSON.

The key-value in the default section will be overwritten by servers section's key-value.
For instance, you can use this option for Azure ResourceGroup name, Azure VM Name and so on

• config.toml

[default]
optional = [
    ["key1", "default_value"],
    ["key3", "val3"],
]

[servers.bsd]
host     = "192.168.11.11"
user     = "kanbe"
optional = [
    ["key1", "val1"],
    ["key2", "val2"],
]

• all.json

[
  {
    "ServerName": "bsd",
    "Family": "FreeBSD",
    "Release": "10.3-RELEASE",
    .... snip ...
    "Optional": [
      [  "key1", "val1" ],
      [  "key2", "val2" ],
      [  "key3", "val3" ]
    ]
  }
]

• FIx Source
• FIx README