Move style and formatter API into chroma package.

formatter.go

@@ -0,0 +1,18 @@
+package chroma
+
+import (
+	"io"
+)
+
+// A Formatter for Chroma lexers.
+type Formatter interface {
+	// Format returns a formatting function for tokens.
+	Format(w io.Writer, style *Style) (func(*Token), error)
+}
+
+// A FormatterFunc is a Formatter implemented as a function.
+type FormatterFunc func(io.Writer, *Style) (func(*Token), error)
+
+func (f FormatterFunc) Format(w io.Writer, s *Style) (func(*Token), error) {
+	return f(w, s)
+}