Add basename function to sgml FuncMap

This function is necessary to allow custom backends to only use the
basename of an image in cases where images are stored as attachments.

blockImageTmpl = `<image ><attachment filename="{{ basename .Src }}" /></image>`

pkg/renderer/sgml/sgml.go

@@ -3,6 +3,7 @@ package sgml
 import (
 	"fmt"
 	"io"
+	"path/filepath"
 	"strings"
 	texttemplate "text/template"
 
@@ -19,6 +20,7 @@ func Render(doc *types.Document, config *configuration.Configuration, output io.
 		templates: tmpls,
 		// Establish some default function handlers.
 		functions: texttemplate.FuncMap{
+			"basename":           filepath.Base,
 			"escape":             escapeString,
 			"halign":             halign,
 			"lastInStrings":      lastInStrings,