Allow customizing reading of #+INCLUDE files

It's not always sensible to allow #+INCLUDE of arbitrary files (e.g. user
provided content)

org/document.go

@@ -27,6 +27,7 @@ type Configuration struct {
 	AutoLink            bool                                  // Try to convert text passages that look like hyperlinks into hyperlinks.
 	DefaultSettings     map[string]string                     // Default values for settings that are overriden by setting the same key in BufferSettings.
 	Log                 *log.Logger                           // Log is used to print warnings during parsing.
+	ReadFile            func(filename string) ([]byte, error) // ReadFile is used to read e.g. #+INCLUDE files.
 }
 
 // Document contains the parsing results and a pointer to the Configuration.
@@ -83,6 +84,7 @@ func New() *Configuration {
 			"OPTIONS":      "toc:t <:t e:t f:t pri:t todo:t tags:t",
 		},
 		Log:      log.New(os.Stderr, "go-org: ", 0),
+		ReadFile: ioutil.ReadFile,
 	}
 }
 

org/keyword.go

@@ -127,7 +127,7 @@ func (d *Document) newInclude(k Keyword) (int, Node) {
 			path = filepath.Join(filepath.Dir(d.Path), path)
 		}
 		resolve = func() Node {
-			bs, err := ioutil.ReadFile(path)
+			bs, err := d.ReadFile(path)
 			if err != nil {
 				d.Log.Printf("Bad include %#v: %s", k, err)
 				return k