Find configuration file in XDG Base Directory

Find configuration file in XDG Base Directory only if `PRYRC` is not
set, and `~/.pryrc` does not exist, therefore not confusing current
users. Note that cache file path is not modified.

lib/pry/pry_class.rb

@@ -1,7 +1,18 @@
 require 'pry/config'
 class Pry
 
-  HOME_RC_FILE = ENV["PRYRC"] || "~/.pryrc"
+  HOME_RC_FILE =
+    if ENV.key?('PRYRC')
+      ENV['PRYRC']
+    elsif File.exist?(File.expand_path('~/.pryrc'))
+      '~/.pryrc'
+    elsif ENV.key?('XDG_CONFIG_HOME') && ENV['XDG_CONFIG_HOME'] != ''
+      # See XDG Base Directory Specification at
+      # https://standards.freedesktop.org/basedir-spec/basedir-spec-0.8.html
+      ENV['XDG_CONFIG_HOME'] + '/pry/pryrc'
+    else
+      '~/.config/pry/config'
+    end
   LOCAL_RC_FILE = "./.pryrc"
 
   class << self