Add yarn option

[calebeby]

See #234

[calebeby]

Hm. The yarn install issue in the first build must have been a network issue.

[jescalan]

I know this might be difficult, but it would be great to add a test for this. I think you'd have to make some small changes to the travis config file to get it installed properly.

[calebeby]

I figured you'd ask for tests :)

So you want it to run spike new in the tests and check if node_modules exists?

[jescalan]

You can just check if it compiles -- if node_modules doesnt exist, you know it's not working. There's already a test for the new command in there, you'd just have to add another with this flag 😁