Special case Comma(math.MinInt64).

The lowest value of an int64 is itself when negated, so the rest of
the code kind of breaks down.  Short circuit it to avoid all the problems.

https://play.golang.org/p/AuAGwhJ69d

Closes: #45

comma.go

@@ -2,6 +2,7 @@ package humanize
 
 import (
 	"bytes"
+	"math"
 	"math/big"
 	"strconv"
 	"strings"
@@ -13,6 +14,12 @@ import (
 // e.g. Comma(834142) -> 834,142
 func Comma(v int64) string {
 	sign := ""
+
+	// minin64 can't be negated to a usable value, so it has to be special cased.
+	if v == math.MinInt64 {
+		return "-9,223,372,036,854,775,808"
+	}
+
 	if v < 0 {
 		sign = "-"
 		v = 0 - v

comma_test.go

@@ -20,6 +20,8 @@ func TestCommas(t *testing.T) {
 		{"10,001,000", Comma(10001000), "10,001,000"},
 		{"123,456,789", Comma(123456789), "123,456,789"},
 		{"maxint", Comma(9.223372e+18), "9,223,372,000,000,000,000"},
+		{"math.maxint", Comma(math.MaxInt64), "9,223,372,036,854,775,807"},
+		{"math.minint", Comma(math.MinInt64), "-9,223,372,036,854,775,808"},
 		{"minint", Comma(-9.223372e+18), "-9,223,372,000,000,000,000"},
 		{"-123,456,789", Comma(-123456789), "-123,456,789"},
 		{"-10,100,000", Comma(-10100000), "-10,100,000"},