feat(component): add FileUploader

Closes #16

src/components/FileUploader/FileUploaderButton.svelte

@@ -0,0 +1,64 @@
+<script>
+  let className = undefined;
+  export { className as class };
+  export let disableLabelChanges = false;
+  export let id = Math.random();
+  export let labelText = 'Add file';
+  export let multiple = false;
+  export let name = '';
+  export let role = 'button';
+  export let tabindex = '0';
+  export let kind = 'primary';
+  export let accept = [];
+  export let disabled = false;
+  export let style = undefined;
+
+  import { cx } from '../../lib';
+
+  const _class = cx(
+    '--btn',
+    '--btn--sm',
+    kind && `--btn--${kind}`,
+    disabled && '--btn--disabled',
+    className
+  );
+
+  let inputRef = undefined;
+</script>
+
+<label
+  tabindex={disabled ? '-1' : tabindex}
+  aria-disabled={disabled}
+  class={_class}
+  for={id}
+  on:keydown
+  on:keydown={({ key }) => {
+    if (key === ' ' || key === 'Enter') {
+      inputRef.click();
+    }
+  }}
+  {style}>
+  <span {role}>{labelText}</span>
+</label>
+<input
+  bind:this={inputRef}
+  type="file"
+  tabindex="-1"
+  class={cx('--visually-hidden')}
+  on:change|stopPropagation
+  on:change|stopPropagation={({ target }) => {
+    const files = target.files;
+    const length = files.length;
+    if (files && !disableLabelChanges) {
+      labelText = length > 1 ? `${length} files` : files[0].name;
+    }
+  }}
+  on:click
+  on:click={event => {
+    event.target.value = null;
+  }}
+  {id}
+  {disabled}
+  {multiple}
+  {accept}
+  {name} />