Event source mapping support

[starlightslo]

I created a new JSON file (event_sources.json) for event source mapping feature and added a new option (-S) on deploy command for a specific event source mapping file.

[
    {
        "EventSourceArn": "your event source arn",
        "StartingPosition": "LATEST",
        "BatchSize": 100,
        "Enabled": true
    }
]

This commit not only provides create event source mapping but also can according to the existing event sources to update or delete. If you think this commit is good, please help me to merge or let me know how can I improve it. Thanks.

[DeviaVir]

It looks good! I will do some more testing before merging, thank you for your contribution and your patience with this PR 👍

[DeviaVir]

This change is now live and published to NPM as 0.8.14.

[liammclennan]

This is a breaking change. I updated my version of node-lambda and deploy now crashes with

Error: ENOENT: no such file or directory, open 'event_sources.json'
    at Error (native)
    at Object.fs.openSync (fs.js:640:18)
    at Object.fs.readFileSync (fs.js:508:33)
    at Object.readFileSync (/home/liam/work/sas/gecko-integration/node_modules/jsonfile/index.js:66:20)
    at Lambda._eventSourceList (/home/liam/work/sas/gecko-integration/node_modules/node-lambda/lib/main.js:141:28)
    at /home/liam/work/sas/gecko-integration/node_modules/node-lambda/lib/main.js:561:33
    at /home/liam/work/sas/gecko-integration/node_modules/node-lambda/lib/main.js:273:5
    at ChildProcess.exithandler (child_process.js:197:7)
    at emitTwo (events.js:106:13)
    at ChildProcess.emit (events.js:191:7)

during the step => Reading event source file to memory

[abetomo]

It seems necessary to run node-lambda setup and create event_sources.json.
It opened PR(#205) which does not cause an error even if event_sources.json does not exist as before.