Allow configuration to be read from package.json

[Amndeep7]

In reference to DavidAnson/markdownlint#332, you said you'd be open to adding support for reading markdownlint-cli's config from package.json. Would you be willing to provide this functionality?

I believe this would be a similar effort to #85, which looked to add support for an external js config file.

[spyke113]

I also need this feature. How about davidtheclark/cosmiconfig?

[guoyunhe]

This feature will be awesome. You won't want to have tons of config files at project root. package.json is a good place for these configs.

[DavidAnson]

FYI, this is possible now with markdownlint-cli2: https://github.com/DavidAnson/markdownlint-cli2