Exclude web-ext-artifacts folder from web-ext build

[callahad]

When running:

web-ext build

The web-ext-artifacts directory can be completely ignored. Currently it is not ignored. You can see it included by checking the listing inside the .zip. Here is an example using beatify.

$ unzip -l web-ext-artifacts/beastify-1.0.zip
Archive:  web-ext-artifacts/beastify-1.0.zip
  Length     Date   Time    Name
 --------    ----   ----    ----
        0  09-21-16 18:01   beasts/
        0  09-21-16 18:01   content_scripts/
        0  09-21-16 18:01   icons/
        0  09-21-16 18:01   popup/
        0  09-21-16 18:01   web-ext-artifacts/
        ...

[kumar303]

I'm not sure if we have an issue for it but there is a patch in the works: #275

[kumar303]

The contributor working on #275 told me he doesn't currently have time to complete the work so this bug is available for anyone else.

[kumar303]

Actually, this isn't such a good first bug because it involves making some architectural changes to FileFilter.

This isn't very high priority because the problematic files inside web-ext-artifacts (such as other XPI files) are already ignored.

[kumar303]

oops, this was a dupe of #186