add composer support

[MDevster]

No description provided.

[martijngastkemper]

Can you explain why it would be useful to have Composer support. It's a PHP dependency manager, why loading Javascript with it?

[MDevster]

We are currently developing a social feed module for the content management system Contao and want to use your jQuery plugin for the presentation. Since Contao 4, the extensions are only made available via Composer or via a special Contao Manager. We would not have to include the additional Javascripts in our bundle, we could simply define the dependencies. See project composer.json (now is forked)

[martijngastkemper]

I still don't see a way Composer is useful for Javascript sources, but I'm willing to merge the PR. Can you change the package name back to pavelk2/social-feed. The name will match the repo and it's more easy to find it in Packagist.