Stop to read unused source code

Before this commit all source code were loaded, even if
they are not used.
For example when taking test coverage for rails applications,
in many cases files on "vendor/bundle" are filtered out.
By reading source code lazily, the time required to generate
results will be shortened.

lib/simplecov/source_file.rb

@@ -74,16 +74,18 @@ def status
     attr_reader :filename
     # The array of coverage data received from the Coverage.result
     attr_reader :coverage
-    # The source code for this file. Aliased as :source
-    attr_reader :src
-    alias source src
 
     def initialize(filename, coverage)
       @filename = filename
       @coverage = coverage
-      File.open(filename, "rb") { |f| @src = f.readlines }
     end
 
+    # The source code for this file. Aliased as :source
+    def src
+      @src ||= File.open(filename, "rb", &:readlines)
+    end
+    alias source src
+
     # Returns all source lines for this file as instances of SimpleCov::SourceFile::Line,
     # and thus including coverage data. Aliased as :source_lines
     def lines