Add "engines" specification to "package.json"

[plroebuck]

Problem Description

This package does not indicate which versions of Node are supported.
For example, your update of "markdownlint" caused the supported version
of Node to change in a patch release. This requires others to have to track
through your commits as well as those of your dependencies to figure out
WTH happened. While I see no reason for this package to have specified a
newer version of "markdownlint" (since its API did not change), this change
would have been caught by TravisCI if "engines" had been specified.

This fix should officially update the package version to 2.0.0, as should
have occurred for 1.1.5.

Fix to specify Node-6 LTS

...
"engines": {
    "node": ">= 6.14.12",
    "npm": ">= 3.10.10"
  }
}

[sagiegurari]

done