Pagination, readable error messages to user, syntax highlighting started.

Pagination now works. Temporary hardcoded 10 pastes per page, will be put
in configuration later. Maybe.

From now user will receive readable error message if error occured.

Started to work on syntax highlighting, tried to make lexers detection
work but apparently to no avail.

assets/index.html

@@ -72,6 +72,16 @@
                             </div>
                         </div>
                     </div>
+                    <div class="field">
+                        <label for="paste-language">Paste should be available for:</label>
+                        <div class="control">
+                            <div class="select">
+                                <select id="paste-language" name="paste-language">
+                                    {lexers}
+                                </select>
+                            </div>
+                        </div>
+                    </div>
                     <div class="field">
                         <div class="control">
                             <input class="button is-success" type="submit" value="Paste!">