Expose the pattern engines as a property of the patternlab object

[bleathem]

When developing a patternlab-node plugin, I found it useful to have access to the loaded pattern engines as a property of the patternlab object. This PR introduces an engines property to the patternlab object.

[bmuenzenmeyer]

@geoffp pattern engines are your baby - see anything wrong with this? I am cool

[bleathem]

Here's an example of how I consume the patternlab.engine property in my patternlab-node plugin:
https://github.com/bleathem/patternlab-plugin-handlebars-helper-listitems/blob/master/index.js#L13

[bmuenzenmeyer]

@geoffp poke

[geoffp]

I have received the poke! I'll check into this.

[geoffp]

Yeah, I'm cool with it. I had designed the pattern_engines module to be addressable as an array of engines when you require it, just the way I think you want, but being that it's part of patternlab-node, I can see how that would be super awkward from inside a plugin. No harm in hanging another reference on the main PL object.

[bmuenzenmeyer]

This will be released with next version - thanks!