Added `onRender` callback

This pull request adds a callback for `onRender.` This callback would fire every time the calendar is rendered, both when prev/next arrow are clicked as well as the initial render.

We use pickadate and push some custom header text into the DOM. It became apparent that we needed a hook everytime the calendar was rendered to repush that text, but we currently only had the ability to push it whent he calendar was first opened.

I didn't minify the JS because I couldn't figure out which tool you were using to do that. If you want me to minify it, I can do that. I also took the liberty to do a minor version bump, so I hope you don't mind. There were no notes in the **Contributing** section about how you like to handle that.

Thanks!
@andrewpthorp