Add `eventLayout` config option

[ZachHaber]

closes #1

eventLayout lets you have fine-grained control over what you are sending to the logfaces server.

For example, if you want relative file path names being sent in f instead of absolute:

log4js.configure({
  appenders: {
    logfaces: {
      type: "@log4js-node/logfaces-http",
      url: "http://lfs-server/logs",
      eventLayout: (event, lfEvent)=>{
        if(lfEvent.f){
          lfEvent.f = path.relative(path.resolve(), lfEvent.f);
        }
        return lfEvent;
      }
    },
  },
  categories: {
    default: { appenders: ["logfaces"], level: "info", enableCallStack: true },
  },
});

Or if you want to add commas between separate data entries:

{
  eventLayout: (event, lfEvent)=>({
    ...lfEvent,
    m: event.data.map(item=>util.format(item)).join(', ');
  })
}

[lamweili]

I made some changes to the commits:

• removed 21378ba3947df5cf800cce43c53e13d25b63cc71 and 1aa644759551f5678e31ad16a58ebe6a1d4dc877 as they are not related to eventLayout (will create another PR for them)
• rebased to master as it merged #33 and #30
• added 574447f64e01f3b5de7608cbde39b33b140826a9 to remove the eslint rules override

[lamweili]

@ZachHaber This PR should be good to go once the review comments are addressed. 🤗

[lamweili]

@ZachHaber Just a little more miscellaneous changes.

I tried to include most in suggestions so that you can Commit suggestion or Add suggestion to batch to make your life easier. There is one for n which I'm not exactly sure the type which may require a manual commit.