Change nsi template from using association.ext to association.extensions, to match struct field in FileAssociation.

[trepidacious]

This allows file associations to be generated in .nsi files, and therefore in the final NSIS installer.

Fixes #364

[FabianLars-crabnebula]

Thanks for the PR! I know the change is fairly small but i need all commits of a PR signed to be able to merge it (i don't have admin permissions like Lucas in your last PR to overwrite that here, not that i'd want to). Also, a small changefile similar to https://github.com/mountainlizard/cargo-packager/blob/52b12e3aad2805af79c49a5ea3133280a1576e83/.changes/notarize-keychain.md would be appreciated.

Thanks again :)

[trepidacious]

You're very welcome, thanks for the pointers - I've amended the original commit to be signed, and added a change file as requested.
Quick query on the notarize-keychain.md file, would it technically be a minor change rather than a patch, since it introduces a new (backwards compatible) feature?

[FabianLars-crabnebula]

yes, in my eyes a minor would have been more appropriate since the public api was affected

[denjell-crabnebula]

Thankyou @trepidacious