Support hard tabs

Now, the highlight line is created by replacing non-tab characters with
spaces, and keeping all hard tabs as-is. This means the highlight line
has the completely same indentation as the code snippet line.

Fixes #7

lib/error_highlight/formatter.rb

@@ -3,7 +3,8 @@ class DefaultFormatter
     def message_for(spot)
       # currently only a one-line code snippet is supported
       if spot[:first_lineno] == spot[:last_lineno]
-        marker = " " * spot[:first_column] + "^" * (spot[:last_column] - spot[:first_column])
+        indent = spot[:snippet][0...spot[:first_column]].gsub(/[^\t]/, " ")
+        marker = indent + "^" * (spot[:last_column] - spot[:first_column])
 
         "\n\n#{ spot[:snippet] }#{ marker }"
       else

test/test_error_highlight.rb

@@ -1,6 +1,7 @@
 require "test/unit"
 
 require "error_highlight"
+require "tempfile"
 
 class ErrorHighlightTest < Test::Unit::TestCase
   class DummyFormatter
@@ -999,4 +1000,20 @@ def custom_formatter.message_for(spot)
   ensure
     ErrorHighlight.formatter = original_formatter
   end
+
+  def test_hard_tabs
+    tmp = Tempfile.new(["error_highlight_test", ".rb"])
+    tmp << "\t \t1.time {}\n"
+    tmp.close(false)
+
+    assert_error_message(NoMethodError, <<~END.gsub("_", "\t")) do
+undefined method `time' for 1:Integer
+
+_ _1.time {}
+_ _ ^^^^^
+    END
+
+      load tmp.path
+    end
+  end
 end