config: make config file system pluggable

If you are using rclone a library you can decide to use the rclone
config file system or not by calling

    configfile.LoadConfig(ctx)

If you don't you will need to set `config.Data` to an implementation
of `config.Storage`.

Other changes
- change interface of config.FileGet to remove unused default
- remove MustValue from config.Storage interface
- change GetValue to return string or bool like elsewhere in rclone
- implement a default config file system which panics with helpful error
- implement getWithDefault to replace the removed MustValue
- don't embed goconfig.ConfigFile so we can change the methods

cmd/mountlib/rc_test.go

@@ -13,6 +13,7 @@ import (
 	_ "github.com/rclone/rclone/cmd/cmount"
 	_ "github.com/rclone/rclone/cmd/mount"
 	_ "github.com/rclone/rclone/cmd/mount2"
+	"github.com/rclone/rclone/fs/config/configfile"
 	"github.com/rclone/rclone/fs/rc"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -20,6 +21,7 @@ import (
 
 func TestRc(t *testing.T) {
 	ctx := context.Background()
+	configfile.LoadConfig(ctx)
 	mount := rc.Calls.Get("mount/mount")
 	assert.NotNil(t, mount)
 	unmount := rc.Calls.Get("mount/unmount")