Support $$ for escaping $ in interpolated values.

Fixes #248

interpolate.go

@@ -5,7 +5,7 @@ import (
 	"regexp"
 )
 
-var interpolationRegex = regexp.MustCompile(`((?:\${([[:alpha:]_][[:word:]]*))(?:=([^}]+))?})|(\$)|([^$]+)`)
+var interpolationRegex = regexp.MustCompile(`(\$\$)|((?:\${([[:alpha:]_][[:word:]]*))(?:=([^}]+))?})|(\$)|([^$]+)`)
 
 // Interpolate variables from vars into s for substrings in the form ${var} or ${var=default}.
 func interpolate(s string, vars Vars, updatedVars map[string]string) (string, error) {
@@ -21,14 +21,16 @@ func interpolate(s string, vars Vars, updatedVars map[string]string) (string, er
 		}
 	}
 	for _, match := range matches {
-		if name := match[2]; name != "" {
+		if dollar := match[1]; dollar != "" {
+			out += "$"
+		} else if name := match[3]; name != "" {
 			value, ok := vars[name]
 			if !ok {
 				// No default value.
-				if match[3] == "" {
+				if match[4] == "" {
 					return "", fmt.Errorf("undefined variable ${%s}", name)
 				}
-				value = match[3]
+				value = match[4]
 			}
 			out += value
 		} else {

interpolate_test.go

@@ -13,7 +13,7 @@ func TestInterpolate(t *testing.T) {
 	updatedVars := map[string]string{
 		"height": "180",
 	}
-	actual, err := interpolate("${name=Bobby Brown} is ${age} years old and ${height} cm tall", vars, updatedVars)
+	actual, err := interpolate("${name=Bobby Brown} is ${age} years old and ${height} cm tall and likes $${AUD}", vars, updatedVars)
 	require.NoError(t, err)
-	require.Equal(t, `Bobby Brown is 35 years old and 180 cm tall`, actual)
+	require.Equal(t, `Bobby Brown is 35 years old and 180 cm tall and likes ${AUD}`, actual)
 }