resource: Add front matter metadata to Resource

This commit expands the Resource interface with 3 new methods:

* Name
* Title
* Params

All of these can be set in the Page front matter. `Name` will get its default value from the base filename, and is the value used in the ByPrefix and GetByPrefix lookup methods.

Fixes #4244

magefile.go

@@ -231,6 +231,9 @@ func TestCoverHTML() error {
 		}
 		b, err := ioutil.ReadFile(cover)
 		if err != nil {
+			if os.IsNotExist(err) {
+				continue
+			}
 			return err
 		}
 		idx := bytes.Index(b, []byte{'\n'})