CommonJS modularize for npm

[tomoki1207]

Related to this.

• add "main" and "style" sections for package.json
• toggle jquery object passed this plugin depends on the module system
• improve .gitignore

It might contains typo because not testing. :P

[ryoichi-obara]

Thanks for PR.
How will "style" attribute (in package.json) work?

[tomoki1207]

"main" and "style" tells that file path to js and css to some bundlers(browserify, parcelify, ...).
In case of using jQuery plugins via npm, we often bundle these pakages(includes plugins) to one file.