Allow Pathname objects for Stackprof :out

The code for `stackprof_results` currently only does checks to see if
the VALUE for `_stackprof.out` is a `T_STRING`, otherwise it assumes it
is an IO object.

This updates the code to first check if `Pathname` has been required and
defined.  If it is and the `_stackprof.out` is a `Pathname`, then open a
file in the same manner as if it was a string, but calls `.to_s` to
convert it to a ruby string first.

Fixes #79

test/test_stackprof.rb

@@ -2,6 +2,7 @@
 require 'stackprof'
 require 'minitest/autorun'
 require 'tempfile'
+require 'pathname'
 
 class StackProfTest < MiniTest::Test
   def test_info
@@ -180,6 +181,19 @@ def test_out_to_path_string
     refute_empty profile[:frames]
   end
 
+  def test_pathname_out
+    tmpfile  = Tempfile.new('stackprof-out')
+    pathname = Pathname.new(tmpfile.path)
+    ret = StackProf.run(mode: :custom, out: pathname) do
+      StackProf.sample
+    end
+
+    assert_equal tmpfile.path, ret.path
+    tmpfile.rewind
+    profile = Marshal.load(tmpfile.read)
+    refute_empty profile[:frames]
+  end
+
   def math
     250_000.times do
       2 ** 10