* new command line tool: install_plugin.php

* can be used to install plugins from command line. Usage: "php install_plugin.php path_to_file"
* fixed if HTTP_HOST not set in common.php when executed from command line
* Added 'Are you sure?' popup when uninstalling plugin
* Moved plugin install logic to a new class: Plugins
* added some warnings when plugin json file is incomplete

system/templates/admin.plugins.html.twig

@@ -15,7 +15,7 @@
 		<td>{{ plugin.version }}</td>
 		<td>{{ plugin.author }}</td>
 		<td>{{ plugin.contact }}</td>
-		<td><a href="?p=plugins&uninstall={{ plugin.file }}">Uninstall</a></td>
+		<td><a href="?p=plugins&uninstall={{ plugin.file }}" onclick="return confirm('Are you sure?');">Uninstall</a></td>
 	</tr>
 	{% endfor %}
 </table>