Add the ability to extend and add your own tasks

[ben-rogerson]

Thanks to @samstevens for this great idea.

Basically you'd be able to add additional tasks to Swiff yourself.
You'd also be able to customise the menu to show only the tasks you'd use.
Perhaps being able to npm install -g swiff-plugin for the more popular extensions?

Open to suggestions/ideas.

[ben-rogerson]

Might shelve this one for the time being - it'll take a huge refactor to achieve.