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/cmd.go

@@ -25,6 +25,7 @@ import (
 	"github.com/rclone/rclone/fs"
 	"github.com/rclone/rclone/fs/accounting"
 	"github.com/rclone/rclone/fs/cache"
+	"github.com/rclone/rclone/fs/config/configfile"
 	"github.com/rclone/rclone/fs/config/configflags"
 	"github.com/rclone/rclone/fs/config/flags"
 	"github.com/rclone/rclone/fs/filter"
@@ -382,6 +383,9 @@ func initConfig() {
 	// Finish parsing any command line flags
 	configflags.SetFlags(ci)
 
+	// Load the config
+	configfile.LoadConfig(ctx)
+
 	// Hide console window
 	if ci.NoConsole {
 		terminal.HideConsole()