Drop description lines along with brew lines

Also move the describe switch after everything else to be more
consistent with `brew bundle dump`. This doesn't affect help output
ordering.

cmd/add.rb

@@ -24,15 +24,16 @@ def add_args
 
             switch "--formula", "--formulae", description: "Treat all named arguments as formulae."
             switch "--cask", "--casks", description: "Treat all named arguments as casks."
-            switch "--describe",
-                env:         :bundle_dump_describe,
-                description: "`add` adds a description comment above each line, unless the " \
-                             "package does not have a description. " \
-                             "This is enabled by default if HOMEBREW_BUNDLE_DUMP_DESCRIBE is set."
 
             conflicts "formula", "cask"
 
             named_args [:formula, :cask], min: 1
+
+            switch "--describe",
+                env:         :bundle_dump_describe,
+                description: "A description comment is added above each line, unless the " \
+                             "package does not have a description. " \
+                             "This is enabled by default if HOMEBREW_BUNDLE_DUMP_DESCRIBE is set."
         end
     end
 
@@ -106,6 +107,7 @@ def add
                     # Adapted from `BrewDumper.dump`:
                     # https://github.com/Homebrew/homebrew-bundle/blob/master/lib/bundle/brew_dumper.rb#L59-L64
                     brewline = if args.describe? && brew.desc
+                        puts "got env #{ENV.fetch('HOMEBREW_BUNDLE_DUMP_DESCRIBE', nil)}"
                         brew.desc.split("\n").map { |s| "# #{s}\n" }.join
                     else
                         ""

cmd/drop.rb

@@ -78,14 +78,28 @@ def drop
                     brew.full_name
                 end
 
+                # Adapted from `BrewDumper.dump`:
+                # https://github.com/Homebrew/homebrew-bundle/blob/master/lib/bundle/brew_dumper.rb#L59-L64
+                desc = if brew.desc
+                    brew.desc.split("\n").map { |s| "# #{s}\n" }
+                else
+                    ""
+                end
+
                 lines = []
                 has_removed_line = false
 
-                File.foreach(brewfile) do |line|
+                File.foreach(brewfile).each_with_index do |line, i|
                     unless line.match(/^\s*#{brewfile_prefix_type}\s+["'](#{regex_name})["']/)
                         lines.push(line)
                     else
                         has_removed_line = true
+
+                        # This might not be the best way to do this, but it works
+                        # as long as the user never modified the description line.
+                        if lines[-desc.length..-1].eql? desc
+                            lines.pop(desc.length)
+                        end
                     end
                 end