Make --debug set copy-webpack-plugin's 'debug' option

[edmorley]

Setting copy-webpack-plugin's debug option to true increases the log level from warning to info:
https://github.com/webpack-contrib/copy-webpack-plugin#available-options

Which gives output of form:

[copy-webpack-plugin] processing from: '**/*' to: 'static'
[copy-webpack-plugin] begin globbing '...\src\static\**\*' with a context of '...\src\static'
[copy-webpack-plugin] determined that '.../src/static/foo.txt' should write to 'static/foo.txt'
[copy-webpack-plugin] reading ...\src\static\foo.txt to write to assets
[copy-webpack-plugin] writing 'static/foo.txt' to compilation assets from '...\src\static\foo.txt'

Refs #389.

[eliperelman]

Looks good!