Add generic customization to landing page

Allow generic additional HTML or CSS to be added to the landing page
output.
* Fix broken placeholder template.

Signed-off-by: SuperQ <superq@gmail.com>

web/landing_page.css

@@ -15,3 +15,4 @@ label {
   display: inline-block;
   width: {{.Form.Width}}em;
 }
+{{.ExtraCSS}}

web/landing_page.go

@@ -33,6 +33,8 @@ type LandingConfig struct {
 	Description string         // A short description about the exporter.
 	Form        LandingForm    // A POST form.
 	Links       []LandingLinks // Links displayed on the landing page.
+	ExtraHTML   string         // Additional HTML to be embedded.
+	ExtraCSS    string         // Additional CSS to be embedded.
 	Version     string         // The version displayed.
 }
 

web/landing_page.html

@@ -23,12 +23,13 @@ <h1>{{.Name}}</h1>
       <div>
       <form action="{{ .Form.Action}}">
       {{ range .Form.Inputs }}
-      <label>{{ .Label }}:</label>&nbsp;<input type="{{ .Type }}" name="{{ .Name }}" placeholder=" .Placeholder }}" value="{{ .Value }}"><br>
+      <label>{{ .Label }}:</label>&nbsp;<input type="{{ .Type }}" name="{{ .Name }}" placeholder="{{ .Placeholder }}" value="{{ .Value }}"><br>
       {{ end }}
       <input type="submit" value="Submit">
       </form>
       </div>
       {{ end }}
+      {{ .ExtraHTML }}
     </main>
   </body>
 </html>