[Feature] Add Spotify shortcode

[istek]

请求增加spotify短代码，方法如下:

在 layouts/shortcodes/ 增加 spotify.html:

<!--
Parameters:
    type - (Required) album / track / playlist / artist
    id - (Required) Target ID
    width - (Optional) width
    height - (Optional) height
-->

{{ if .IsNamedParams }}
<iframe src="https://open.spotify.com/embed/{{ .Get "type" }}/{{ .Get "id" }}"
    width="{{ default "100%" (.Get "width") }}"
    height="{{ default "380" (.Get "height") }}"
    frameborder="0"
    allowtransparency="true"
    allow="encrypted-media"></iframe>
{{ else }}
<iframe src="https://open.spotify.com/embed/{{ .Get 0 }}/{{ .Get 1 }}"
    width="{{ default "100%" (.Get 2) }}"
    height="{{ default "380" (.Get 3) }}"
    frameborder="0"
    allowtransparency="true"
    allow="encrypted-media"></iframe>
{{ end }}

Useage:
type：必填，可以是 album(专辑)，trace(单曲), playlist (歌单), artist (歌手)
id: 必填， 专辑/单曲/歌单/歌手的目标ID，通过分享可得
width: 宽度
height: 高度

{{< spotify type="album" id="5vqILcEeCT0N1DeJBsLrGE" width="100%" height="250" >}}