Fix issues in tcell renderer and Windows build

- Fix display of CJK wide characters
- Fix horizontal offset of header lines
- Add support for keys with ALT modifier, shift-tab, page-up and down
- Fix util.ExecCommand to properly parse command-line arguments
- Fix redraw on resize
- Implement Pause/Resume for execute action
- Remove runtime check of GOOS
- Change exit status to 2 when tcell failed to start
- TBD: Travis CI build for tcell renderer
    - Pending. tcell cannot reliably ingest keys from tmux send-keys

test/test_go.rb

@@ -1058,7 +1058,7 @@ class TestGoFZF < TestBase
   def test_invalid_term
     lines = `TERM=xxx #{FZF}`
     assert_equal 2, $?.exitstatus
-    assert lines.include?('Invalid $TERM: xxx')
+    assert lines.include?('Invalid $TERM: xxx') || lines.include?('terminal entry not found')
   end
 
   def test_invalid_option