Add default options filename as constant

lib/github_changelog_generator/parser_file.rb

@@ -4,20 +4,21 @@ module GitHubChangelogGenerator
   ParserError = Class.new(StandardError)
 
   class ParserFile
+    FILENAME = ".github_changelog_generator"
+
     def initialize(options)
       @options = options
     end
 
+    # Destructively change @options using data in configured options file.
     def parse!
-      return unless file.exist?
-
-      file.each_line { |line| parse_line!(line) }
+      file.each_line { |line| parse_line!(line) } if file.exist?
     end
 
     private
 
     def file
-      @file ||= Pathname(File.expand_path(@options[:params_file] || ".github_changelog_generator"))
+      @file ||= Pathname(File.expand_path(@options[:params_file] || FILENAME))
     end
 
     def parse_line!(line)