add `style` field

[turtleDev]

Adding a style field to your npm package makes it compatible with sass-module-importer, which is a nice way to import css into your sass project directly from node_modules without relying on the ugly ../../ imports.

I would also recommend compiling a react-s-full.css file (that includes everything needed to use react-s-alert, including animations), and then set that as the style for your module.

The CSS files are not so big to warrant them being in their separate files, or at least for me, it was counter-intuitive. Either way, I don't think adding a full build could hurt :)

[juliancwirko]

Hi, thanks. I couldn't find time to get back to this tool lately. I hope I'll be able to review this one and others asap :)