Use manage_options instead of edit_plugins capability

According to [the WordPress Codex](http://codex.wordpress.org/Roles_and_Capabilities#edit_plugins), the `edit_plugins` capability is mostly used to determine whether Plugins > Plugin Editor is accessible. On sites that use wp-config.php constants like `DISALLOW_FILE_EDIT` (for instance, all of my production sites), the `edit_plugins` capability is deactivated, meaning I can't configure this plugin without enabling the plugin editor.

This pull request changes the capability to the more sensible manage_options - if the user has the power to change the site title, permalink structure, and other much more destructive changes, they should be able to edit the Twitter configuration.