Move capability into constant and make it configurable

[sebastianpopp]

I need the editor role to be able to edit the redirect setting on a project of mine. Currently, there is no way to change the necessary capability without it being overwritten with the next update. With my pull request, I added a constant for the capability which is set to manage_options by default. By setting

define( 'SIMPLE301REDIRECTS_CAPABILITY', 'edit_pages' );

in your wp-config.php you can change the capability without changing the plugin code. There's no change in behavior for users that do not want to change the capability.