fix #34, #63, #72: ignored props before component connects

[riovir]

Added awareness to CustomElement props in the initialization code. In order to stay backward compatible opted for attributes taking precedence. In scenarios, where users already set conflicting attributes and props, the component's behavior will remain unchanged.

Intends to:

• fix Setting props before mounting the web component causes them to be overwritten by default values #34
• fix Async component doesn't get initial props which are passed as element properties #63
• fix Props availability on mounted hook #72

Note that I also added Jest and in-comment globals to ESLint setup to make it pass on tests.

[hpx7]

This is an important fix -- @kazupon are you the right person to look at this?

[metacurb]

It would be wonderful if this could be looked at! I also greatly need this fix