Snowpack: Exclude cannot be controlled because of deepmerge

[sdemjanenko]

I would like to be able to override the default exclude option. When I attempt to do so the exclude config inside snowpack actually just concats my exclude option onto the default. I would prefer that deepmerge not be the default behavior.

Also it does not appear that the docs indicate that the user's config is deepmerged with the default config.

[sdemjanenko]

I believe i have a fix in #279

[drwpow]

Thanks for the fix!