feat(Select): added format prop

[Florian-Schoenherr]

Gives devs the ability to define how the chosen value is shown (inside the underlying TextField).
Closes: #19

[vercel]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/thecomputerm/svelte-materialify/i6lwds3qr
✅ Preview: https://svelte-materialify-git-patch-2.thecomputerm.vercel.app

[TheComputerM]

Can you please change it to this:

export let format = (val) => val.join(', ');

[TheComputerM]

Thanks 👍