Add nconvert package (#3272)

Limit to x86_64 and i686 architecture only

packages/nconvert.rb

+require 'package'
+
+class Nconvert < Package
+  description 'Command Line Batch utility for images'
+  homepage 'https://www.xnview.com/en/nconvert/'
+  version '6.88'
+  case ARCH
+  when 'x86_64'
+    source_url 'https://download.xnview.com/NConvert-linux64.tgz'
+    source_sha256 '852c9cfefb92ea49670c8bc8fc4364297556066026b773321fe6b13718c57467'
+  when 'i686'
+    source_url 'https://download.xnview.com/NConvert-linux.tgz'
+    source_sha256 '10f576609a982c9d7f4a519c9a49899609b63cea65d1a38b0c6f0c2fda2eb8c1'
+  end
+
+  def self.install
+    system "install -Dm755 nconvert #{CREW_DEST_PREFIX}/bin/nconvert"
+  end
+end