string plugin getWithCache doesn't really cache that much

[zepumph]

While working on phetsims/rosetta#193, I found that almost every string being required from the sim was triggering a loading of the string file. I think this is because all the require loads occur before the text plugin has async come back with the JSON.

A good way to test this problem is to put console.log('url') in the text.get() callback.

[zepumph]

Above I added a way to cache the intermediary state before the first loading of a string file. I'm pretty happy with this change. It is simple, and is a definite improvement in my book. @samreid can you please review.

[samreid]

This looks great and looks like it cures the symptoms described in #730.

Some questions and comments:

• {Object.<url:string, Array.<function>} is missing a >
• Why does action => action && action() check action &&?
• In action => action && action() it seems better to pass through parsed directly like action => action && action(parsed), instead of relying on the closure variable to be correct in the arrow function. Would that work?

[zepumph]

Thanks! I really like the suggestions. How's that?

[samreid]

Looks great, thanks! I'm ready to close if you are.