Introduce source.Filesystem

This provides an abstraction over how files are processed by Hugo.  This
allows for alternatives like CMS systems or Dropbox, etc.

source/filesystem.go

@@ -0,0 +1,72 @@
+package source
+
+import (
+	"io"
+	"os"
+	"path/filepath"
+)
+
+type Input interface {
+	Files() []*File
+}
+
+type File struct {
+	Name     string
+	Contents io.Reader
+}
+
+type Filesystem struct {
+	files      []*File
+	Base       string
+	AvoidPaths []string
+}
+
+func (f *Filesystem) Files() []*File {
+	f.captureFiles()
+	return f.files
+}
+
+func (f *Filesystem) add(name string, reader io.Reader) {
+	f.files = append(f.files, &File{Name: name, Contents: reader})
+}
+
+func (f *Filesystem) captureFiles() {
+
+	walker := func(path string, fi os.FileInfo, err error) error {
+		if err != nil {
+			return nil
+		}
+
+		if fi.IsDir() {
+			if f.avoid(path) {
+				return filepath.SkipDir
+			}
+			return nil
+		} else {
+			if ignoreDotFile(path) {
+				return nil
+			}
+			file, err := os.Open(path)
+			if err != nil {
+				return err
+			}
+			f.add(path, file)
+			return nil
+		}
+	}
+
+	filepath.Walk(f.Base, walker)
+}
+
+func (f *Filesystem) avoid(path string) bool {
+	for _, avoid := range f.AvoidPaths {
+		if avoid == path {
+			return true
+		}
+	}
+	return false
+}
+
+func ignoreDotFile(path string) bool {
+	return filepath.Base(path)[0] == '.'
+}