make .select-single input 100% width and 100% height

fixes my issues with #34, but this should not be merged because:

- [x] prefer if there was native support for `.select-multiple` as #36
- [ ] i think the base CSS could be less opinionated
- [ ] you might want to separate select-multiple and select-single from this repo first. it would make things easier

i'm also not 100% sure the padding is correctly... but it looks beautiful to me