Material Drop Shadow Elevation

This is like [@material/elevation][1], but uses [drop-shadow][2] CSS filter
instead of a box-shadow property. This allows to use it on weird shaped things
like [CSS triangles][3].

``
npm install material-ds-elevation
`

Usage:

`scss
@import "material-ds-elevation/functions";
.my-element {
// Instead of:
// @include mdc-elevation(2);
// Use:
filter: drop-shadow-elevation(2);

&::before {
@include make-me-a-triangle(bottom); // not included :P
}
}
``

TODO

- Add transitions

[1]: https://github.com/material-components/material-components-web/tree/master/packages/mdc-elevation
[2]: https://developer.mozilla.org/en-US/docs/Web/CSS/filter-function/drop-shadow
[3]: https://css-tricks.com/snippets/css/css-triangle/