Allow Configurable Converters on CSV

Lets the user specify which converters to pass to `CSV`, defaulting to `nil` if not present in config. This is specifically so that numeric values in a CSV file can be parsed as something other than strings.

lib/jekyll/readers/data_reader.rb

@@ -59,13 +59,15 @@ def read_data_file(path)
       case File.extname(path).downcase
       when ".csv"
         CSV.read(path,
-                 :headers  => true,
-                 :encoding => site.config["encoding"]).map(&:to_hash)
+                 :converters => site.config["csv_converters"]&.map(&:to_sym),
+                 :headers    => true,
+                 :encoding   => site.config["encoding"]).map(&:to_hash)
       when ".tsv"
         CSV.read(path,
-                 :col_sep  => "\t",
-                 :headers  => true,
-                 :encoding => site.config["encoding"]).map(&:to_hash)
+                 :col_sep    => "\t",
+                 :converters => site.config["csv_converters"]&.map(&:to_sym),
+                 :headers    => true,
+                 :encoding   => site.config["encoding"]).map(&:to_hash)
       else
         SafeYAML.load_file(path)
       end