Provide an easy way to add and remove plugins

[fredck]

Currently, it's a bit cumbersome to add and remove plugins in the editor setting. To do that, we need to manipulate the plugins setting string in the custom configuration file. For example:

// Add a new plugin.
config.plugins += ',myplugin';

// Remove the elementspath plugin.
config.plugins = config.plugins.replace( /(?:^|,)elementspath(?=,|$)/, '' );

This situation, other than being counterintuitive, makes it impossible to set these configurations at editor creation time. We should instead have ways to make it with easy, as this is a common task.

[fredck]

Fixed with [3425].