style: add "default" option for SCSS variables
style: add "default" option for SCSS variables

Now SCSS variables can be overriden, for example:

```scss
$animation-speed: .2s;
$animation-function: ease-in-out;
$backdrop-color: rgba(255, 255, 255, 1);
$header-color: rgba(45, 0, 45, .2);

@import "node_modules/smartphoto/scss/smartphoto.scss";
```
appleple/SmartPhoto,appleple/SmartPhoto