- Conditional format with formula support, relate issue qax-os#75;

- go test and readme update

Author: xuri

README.md

@@ -11,7 +11,7 @@
 
 ## Introduction
 
-Excelize is a library written in pure Golang and providing a set of functions that allow you to write to and read from XLSX files. Support reads and writes XLSX file generated by Microsoft Excel™ 2007 and later. Support save file without losing original charts of XLSX. This library needs Go version 1.8 or later. The full API docs can be seen using go's built-in documentation tool, or online at [godoc.org](https://godoc.org/github.com/xuri/excelize).
+Excelize is a library written in pure Golang and providing a set of functions that allow you to write to and read from XLSX files. Support reads and writes XLSX file generated by Microsoft Excel™ 2007 and later. Support save file without losing original charts of XLSX. This library needs Go version 1.8 or later. The full API docs can be seen using go's built-in documentation tool, or online at [godoc.org](https://godoc.org/github.com/360EntSecGroup-Skylar/excelize).
 
 ## Basic Usage
 

excelize_test.go

@@ -900,7 +900,7 @@ func TestRemoveRow(t *testing.T) {
 func TestConditionalFormat(t *testing.T) {
 	xlsx := NewFile()
 	for j := 1; j <= 10; j++ {
-		for i := 0; i <= 10; i++ {
+		for i := 0; i <= 15; i++ {
 			xlsx.SetCellInt("Sheet1", ToAlphaString(i)+strconv.Itoa(j), j)
 		}
 	}
@@ -937,6 +937,8 @@ func TestConditionalFormat(t *testing.T) {
 	xlsx.SetConditionalFormat("Sheet1", "J1:J10", fmt.Sprintf(`[{"type":"average","criteria":"=","format":%d, "above_average": false}]`, format1))
 	// Data Bars: Gradient Fill.
 	xlsx.SetConditionalFormat("Sheet1", "K1:K10", `[{"type":"data_bar", "criteria":"=", "min_type":"min","max_type":"max","bar_color":"#638EC6"}]`)
+	// Use a formula to determine which cells to format.
+	xlsx.SetConditionalFormat("Sheet1", "L1:L10", fmt.Sprintf(`[{"type":"formula", "criteria":"L2<3", "format":%d}]`, format1))
 	err = xlsx.SaveAs("./test/Workbook_conditional_format.xlsx")
 	if err != nil {
 		t.Log(err)

styles.go

@@ -793,7 +793,7 @@ var validType = map[string]string{
 	"2_color_scale": "2_color_scale",
 	"3_color_scale": "3_color_scale",
 	"data_bar":      "dataBar",
-	"formula":       "expression", // Doesn't support currently
+	"formula":       "expression",
 }
 
 // criteriaType defined the list of valid criteria types.
@@ -2540,6 +2540,7 @@ func (f *File) SetConditionalFormat(sheet, area, formatSet string) {
 		"2_color_scale":   drawCondFmtColorScale,
 		"3_color_scale":   drawCondFmtColorScale,
 		"dataBar":         drawCondFmtDataBar,
+		"expression":      drawConfFmtExp,
 	}
 
 	xlsx := f.workSheetReader(sheet)
@@ -2554,7 +2555,7 @@ func (f *File) SetConditionalFormat(sheet, area, formatSet string) {
 		}
 		// Check for valid criteria types.
 		ct, ok = criteriaType[v.Criteria]
-		if !ok {
+		if !ok && vt != "expression" {
 			continue
 		}
 
@@ -2672,6 +2673,17 @@ func drawCondFmtDataBar(p int, ct string, format *formatConditional) *xlsxCfRule
 	}
 }
 
+// drawConfFmtExp provides function to create conditional formatting rule for
+// expression by given priority, criteria type and format settings.
+func drawConfFmtExp(p int, ct string, format *formatConditional) *xlsxCfRule {
+	return &xlsxCfRule{
+		Priority: p + 1,
+		Type:     validType[format.Type],
+		Formula:  []string{format.Criteria},
+		DxfID:    &format.Format,
+	}
+}
+
 // getPaletteColor provides function to convert the RBG color by given string.
 func getPaletteColor(color string) string {
 	return "FF" + strings.Replace(strings.ToUpper(color), "#", "", -1)