Fix corrupt images

[stevehanson]

There is an issue where the assets images were all corrupted in the generated Belt app. This is occurring because Belt reads files as utf-8 text and then writes them when copying. It isn't handling binary files appropriately. The only binary files I’m aware of right now are in templates/boilerplate/assets.

To reproduce the original issue, from Belt project dir:

node bin/belt.js NewApp
cd builds/NewApp
open assets/splash.png
# corrupted image

This updates the copyTemplateDirectory function to use fs.copy for binary files (currently defined using an array of filenames that we identify as binary, but we might want something more robust in the future) instead of reading the file, optionally transforming it, and then writing it.

[codeofdiego]

Looks great, just moved the list of unsupported extensions to the beginning of the file.

I also ran into an issue where I was still getting the images corrupted but that due to the files in the dist folder not being updated with the remote changes (ignored). But after running yarn build I was able to test it.