Don't determine auto style on init

The automatic style configuration will now only be determined, when
initialized with the WithAutoStyle renderer option.

styles.go

@@ -3,16 +3,10 @@ package glamour
 //go:generate go run ./internal/generate-style-json
 
 import (
-	"github.com/muesli/termenv"
-
 	"github.com/charmbracelet/glamour/ansi"
 )
 
 var (
-	// AutoStyleConfig is either pointing to DarkStyleConfig or LightStyleConfig
-	// depending on the background color of the terminal glamour is run in
-	AutoStyleConfig *ansi.StyleConfig
-
 	// ASCIIStyleConfig uses only ASCII characters.
 	ASCIIStyleConfig = ansi.StyleConfig{
 		Document: ansi.StyleBlock{
@@ -664,12 +658,3 @@ var (
 func boolPtr(b bool) *bool       { return &b }
 func stringPtr(s string) *string { return &s }
 func uintPtr(u uint) *uint       { return &u }
-
-func init() {
-	AutoStyleConfig = &LightStyleConfig
-	if termenv.HasDarkBackground() {
-		AutoStyleConfig = &DarkStyleConfig
-	}
-
-	DefaultStyles["auto"] = AutoStyleConfig
-}