Child process exec options, fixes maxBuffer size problem

[yunusga]

This PR closes #3 and closes #7.

Increase maxBuffer size to prevent #3, !!! OUTPUT ERROR or Unexpected end of JSON input errors. This is because child_process stdout being truncated when validator check a lot of files.

Version	maxBuffer	Documentation link
8.x LTS	200 * 1024	child_process.exec(command[, options][, callback])
10.x LTS	200 * 1024	child_process.exec(command[, options][, callback])
12.x	1024 * 1024	child_process.exec(command[, options][, callback])

[OlehDutchenko]

@yunusga thx a lot!

[OlehDutchenko]

@yunusga - I'am just already updated new version at npm - 1.3.0,
and my thanks in readme.md https://github.com/dutchenkoOleg/node-w3c-validator#contributors-

[yunusga]

@dutchenkoOleg, cool 👍