Add plugins

[treygriffith]

This change adds a new concept of Plugins to ui-box to enable arbitrary userland processing of selectors and styles.

The plugin structure closely resembles that of glamor, which it was inspired by.

In the public API, this change exposes one new function: usePlugin.

usePlugin takes as its sole argument a function which should accept and return an object with a selector property, containing the string selector for a given ruleset, and a rules property, containing an array of rules, each with a property and value.

Plugins are executed in reverse from the order they are added (also from glamor). So if you add plugins a, b, and then c, they will be executed as c->b->a.

The motivation for this change is #74. However, given that a plugin system may open up ui-box further than desired, I'm happy to open another PR which addresses #74 specifically without a generic plugin.

[mshwery]

This looks pretty solid. I'm curious – can we test the unhappy path too? I would love to have to a good story around preventing poorly behaving plugins from completely breaking ui-box.

[treygriffith]

can we test the unhappy path too? I would love to have to a good story around preventing poorly behaving plugins from completely breaking ui-box.

Did you have anything in mind? Only thing I can think of is testing that a plugin returns an object roughly in the shape that we expect. But I think the danger of a plugin system is that bad plugins can break ui-box, difficult to prevent that without severely limiting what they can do.

[mshwery]

@treygriffith if a plugin does not return { selector: string, rules: Rule[] } how will ui-box handle that? Should we throw? Should we catch the error, console it, and move on without that plugin?

[treygriffith]

@mshwery I added some handling for plugins that don't return something that at least looks like what we expect. In production, it skips the plugin and moves on. In all other environments it will throw.

Let me know what you think of that approach. It obviously doesn't handle things that look valid but aren't (e.g. nonsense selectors like $my-div).

[treygriffith]

@mshwery any further thoughts on this PR?