Use manage_options instead of edit_plugins capability

[stevegrunwell]

According to the WordPress Codex, 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.