Add Slack to social options (#236)

exampleSite/config.toml

   gitlab = ""
   linkedin = ""
   mastodon = ""
+  slack = ""
   # choose a background color from any on this page: http://tachyons.io/docs/themes/skins/ and preface it with "bg-"
   background_color_class = "bg-black"
   featured_image = "/images/gohugo-default-sample-hero-image.jpg"

layouts/partials/social-follow.html

   {{- partial "new-window-icon.html" . -}}
 </a>
 {{ end }}
+{{ with .Param "slack" }}
+<a href="{{ . }}" target="_blank" class="link-transition slack link dib z-999 pt3 pt0-l mr1" title="Slack link" rel="noopener" aria-label="follow on Slack——Opens in a new window">
+  {{ partial "svg/slack.svg" (dict "size" $icon_size) }}
+  {{- partial "new-window-icon.html" . -}}
+</a>
+{{ end }}

layouts/partials/svg/slack.svg

+<svg {{ with .size }} height="{{ . }}" {{ end }} style="enable-background:new 0 0 270 270;" version="1.1" viewBox="0 0 270 270" width="{{ .size }}" xml:space="preserve" xmlns="http://www.w3.org/2000/svg">
+<style type="text/css">
+	.st0{fill:#BABABA;}
+</style>
+<g>
+	<g>
+		<path class="st0" d="M99.4,151.2c0,7.1-5.8,12.9-12.9,12.9s-12.9-5.8-12.9-12.9c0-7.1,5.8-12.9,12.9-12.9h12.9V151.2z"/>
+		<path class="st0" d="M105.9,151.2c0-7.1,5.8-12.9,12.9-12.9s12.9,5.8,12.9,12.9v32.3c0,7.1-5.8,12.9-12.9,12.9
+			s-12.9-5.8-12.9-12.9C105.9,183.5,105.9,151.2,105.9,151.2z"/>
+	</g>
+	<g>
+		<path class="st0" d="M118.8,99.4c-7.1,0-12.9-5.8-12.9-12.9s5.8-12.9,12.9-12.9s12.9,5.8,12.9,12.9v12.9H118.8z"/>
+		<path class="st0" d="M118.8,105.9c7.1,0,12.9,5.8,12.9,12.9s-5.8,12.9-12.9,12.9H86.5c-7.1,0-12.9-5.8-12.9-12.9
+			s5.8-12.9,12.9-12.9C86.5,105.9,118.8,105.9,118.8,105.9z"/>
+	</g>
+	<g>
+		<path class="st0" d="M170.6,118.8c0-7.1,5.8-12.9,12.9-12.9c7.1,0,12.9,5.8,12.9,12.9s-5.8,12.9-12.9,12.9h-12.9V118.8z"/>
+		<path class="st0" d="M164.1,118.8c0,7.1-5.8,12.9-12.9,12.9c-7.1,0-12.9-5.8-12.9-12.9V86.5c0-7.1,5.8-12.9,12.9-12.9
+			c7.1,0,12.9,5.8,12.9,12.9V118.8z"/>
+	</g>
+	<g>
+		<path class="st0" d="M151.2,170.6c7.1,0,12.9,5.8,12.9,12.9c0,7.1-5.8,12.9-12.9,12.9c-7.1,0-12.9-5.8-12.9-12.9v-12.9H151.2z"/>
+		<path class="st0" d="M151.2,164.1c-7.1,0-12.9-5.8-12.9-12.9c0-7.1,5.8-12.9,12.9-12.9h32.3c7.1,0,12.9,5.8,12.9,12.9
+			c0,7.1-5.8,12.9-12.9,12.9H151.2z"/>
+	</g>
+</g>
+</svg>

src/css/_social-icons.css

-.facebook, .twitter, .instagram, .youtube, .github, .gitlab, .linkedin, .medium, .mastodon {
+.facebook, .twitter, .instagram, .youtube, .github, .gitlab, .linkedin, .medium, .mastodon, .slack {
   fill: #BABABA;
 }
 
 .mastodon:hover {
   fill: #3088d4;
 }
+
+.slack:hover {
+  fill: #3088d4;
+}
+

stackbit.yaml

           - type: string
             name: mastodon
             label: Mastodon
+          - type: string
+            name: slack
+            label: Slack
           - type: string
             name: background_color_class
             label: background_color_class