Make plugin more friendly to modification

[markjaquith]

I needed to disable the "Primary Term" feature for a client. I couldn't find a way to do it without hacking the plugin to prevent that class from being instantiated. The instance of that feature's class is in a private variable, and there is no filter call that lets you toggle it on or off. Within the class itself, there is no "should I register all my hooks?" filter call. It's just all set in stone, and there's nothing anything external can do about it.

[thulshof]

Thanks for your report. We're providing multiple filters to enable users to change the behavior of our plugin. You can see a list of filters on our website. Currently we're focusing on issues that affect many users. We've concluded that this issue is not experienced by many users, therefore we are not able to fix this short-term. However, if more users are requesting this feature, we'll of course revisit this issue.

[thulshof]

@markjaquith We've just discussed this internally and we'll re-open this issue. However, currently we don't have the capacity to add this feature but you are very welcome to submit a PR that adds a filter for this.

[stodorovic]

Related to #6184. There are more info about this issue.