Add copy-to-clipboard button to meta page

Fixes #42.

assets/templates/html/pasteMeta.html.tmpl

@@ -2,8 +2,15 @@
 '{{.Paste.Key}}{{.FileExt}}' metadata - rushlink
 {{end}}
 
+{{define "head-append"}}
+<script type="text/javascript" src="/js/copyclipboard.js" defer></script>
+{{end}}
+
 {{define "body"}}
 <pre>
+<button onclick="copyToClipboard('{{.RootURL}}/{{.Paste.Key}}{{.FileExt}}')">Copy URL to clipboard</button> <span id="copyToClipboardStatus"></span>
+</pre>
+<pre>
 <a href="{{.RootURL}}/{{.Paste.Key}}{{.FileExt}}">{{.RootURL}}/{{.Paste.Key}}{{.FileExt}}</a>
 ---
 {{if and (ne .Paste.State.String "deleted") .CanDeleteBool}}