Consider case when sorting strings

This makes sorting flags and other sections consistent with how most
command line tools function, by placing both flags `-A` and `-a` before
a flag `-B`.

category.go

@@ -10,7 +10,7 @@ type CommandCategory struct {
 }
 
 func (c CommandCategories) Less(i, j int) bool {
-	return c[i].Name < c[j].Name
+	return lexicographicLess(c[i].Name, c[j].Name)
 }
 
 func (c CommandCategories) Len() int {

command.go

@@ -73,7 +73,7 @@ func (c CommandsByName) Len() int {
 }
 
 func (c CommandsByName) Less(i, j int) bool {
-	return c[i].Name < c[j].Name
+	return lexicographicLess(c[i].Name, c[j].Name)
 }
 
 func (c CommandsByName) Swap(i, j int) {

flag.go

@@ -53,7 +53,7 @@ func (f FlagsByName) Len() int {
 }
 
 func (f FlagsByName) Less(i, j int) bool {
-	return f[i].GetName() < f[j].GetName()
+	return lexicographicLess(f[i].GetName(), f[j].GetName())
 }
 
 func (f FlagsByName) Swap(i, j int) {

sort.go

@@ -0,0 +1,29 @@
+package cli
+
+import "unicode"
+
+// lexicographicLess compares strings alphabetically considering case.
+func lexicographicLess(i, j string) bool {
+	iRunes := []rune(i)
+	jRunes := []rune(j)
+
+	lenShared := len(iRunes)
+	if lenShared > len(jRunes) {
+		lenShared = len(jRunes)
+	}
+
+	for index := 0; index < lenShared; index++ {
+		ir := iRunes[index]
+		jr := jRunes[index]
+
+		if lir, ljr := unicode.ToLower(ir), unicode.ToLower(jr); lir != ljr {
+			return lir < ljr
+		}
+
+		if ir != jr {
+			return ir < jr
+		}
+	}
+
+	return i < j
+}

sort_test.go

@@ -0,0 +1,30 @@
+package cli
+
+import "testing"
+
+var lexicographicLessTests = []struct {
+	i        string
+	j        string
+	expected bool
+}{
+	{"", "a", true},
+	{"a", "", false},
+	{"a", "a", false},
+	{"a", "A", false},
+	{"A", "a", true},
+	{"aa", "a", false},
+	{"a", "aa", true},
+	{"a", "b", true},
+	{"a", "B", true},
+	{"A", "b", true},
+	{"A", "B", true},
+}
+
+func TestLexicographicLess(t *testing.T) {
+	for _, test := range lexicographicLessTests {
+		actual := lexicographicLess(test.i, test.j)
+		if test.expected != actual {
+			t.Errorf(`expected string "%s" to come before "%s"`, test.i, test.j)
+		}
+	}
+}