Show plugin configs in `listconfigs`

[niftynei]

For maximum correctness, listconfigs should show all configs, including plugins.

[m-schmoock]

@niftynei how do you want to have the plugin configurations be formatted in the JSON output?
Note that there is aready a duplicated key "plugin" per plugin in the listconfigs output:

ln_cli listconfigs
{
   "# version": "v0.7.3-126-g654faa6-modded",
   "lightning-dir": "/home/will/.lightning",
   "wallet": "sqlite3:///home/will/.lightning/lightningd.sqlite3",
   "plugin": "/home/will/projects/lightning.git/lightningd/../plugins/autoclean",
   "plugin": "/home/will/projects/lightning.git/lightningd/../plugins/fundchannel",
   "plugin": "/home/will/projects/lightning.git/lightningd/../plugins/pay",
   "plugin": "/home/will/.lightning/plugins/sendinvoiceless/sendinvoiceless.py",

For plugins, we could change the output to something like this:

{
   "# version": "v0.7.3-126-g654faa6-modded",
   "lightning-dir": "/home/will/.lightning",
   "wallet": "sqlite3:///home/will/.lightning/lightningd.sqlite3",
   "plugins": {
      "/home/will/projects/lightning.git/lightningd/../plugins/autoclean": {},
      "/home/will/projects/lightning.git/lightningd/../plugins/fundchannel": {},
      "/home/will/projects/lightning.git/lightningd/../plugins/pay": {},
      "/home/will/.lightning/plugins/sendinvoiceless/sendinvoiceless.py": {
         "cltv-final": "10",
         "fee-base": "null",
         "fee-per-satoshi": "null"
      },
      "/home/will/.lightning/plugins/rebalance/rebalance.py": {
         "cltv-final": "10"
      },
      "/home/will/.lightning/plugins/summary/summary.py": {
         "summary-currency": "USD",
         "summary-currency-prefix": "USD $"
      },
      "/home/will/.lightning/plugins/drain/drain.py": {
         "cltv-final": "10"
      }
   }

[m-schmoock]

Can be closed