Add Gitlab to social icons (#131)

Gitlab is a popular Git repository manager using an opensource license.
This commit adds the Gitlab social icon.

data/webpack_assets.json

 {
   "app": {
     "js": "js/app.3fc0f988d21662902933.js",
-    "css": "css/app.ab4b67a3ea25990fa8279f3b7ef08b61.css"
+    "css": "css/app.e08a958ae3e530145318b6373195c765.css"
   }
 }

exampleSite/config.toml

   instagram = ""
   youtube = ""
   github = ""
+  gitlab = ""
   linkedin = ""
   # 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"

layouts/partials/social-follow.html

     {{ partial "svg/github.svg" (dict "size" $icon_size) }}
   </a>
 {{ end }}
+{{ with .Param "gitlab" }}
+  <a href="{{ . }}" class="link-transition gitlab link dib z-999 pt3 pt0-l mr2" title="Gitlab link">
+    {{ partial "svg/gitlab.svg" (dict "size" $icon_size) }}
+  </a>
+{{ end }}

layouts/partials/svg/gitlab.svg

+<svg {{ with .size }} height="{{ . }}" {{ end }} style="enable-background:new 0 0 512 512;" version="1.1" viewBox="0 0 512 512" width="{{ .size }}" xml:space="preserve" xmlns="http://www.w3.org/2000/svg"><path d="M29.782 199.732L256 493.714 8.074 309.699c-6.856-5.142-9.712-13.996-7.141-21.993l28.849-87.974zm75.405-174.806c-3.142-8.854-15.709-8.854-18.851 0L29.782 199.732h131.961L105.187 24.926zm56.556 174.806L256 493.714l94.257-293.982H161.743zm349.324 87.974l-28.849-87.974L256 493.714l247.926-184.015c6.855-5.142 9.711-13.996 7.141-21.993zm-85.404-262.78c-3.142-8.854-15.709-8.854-18.851 0l-56.555 174.806h131.961L425.663 24.926z"></path></svg>

src/css/_social-icons.css

-.facebook, .twitter, .instagram, .youtube, .github, .linkedin {
+.facebook, .twitter, .instagram, .youtube, .github, .gitlab, .linkedin {
   fill: #BABABA;
 }
 .facebook:hover {
   fill: #6cc644;
 }
 
+.gitlab:hover {
+  fill: #FC6D26;
+}
+
 .linkedin:hover {
   fill: #0077b5
 }