Add support for date placeholder in process config

Because it doesn't make sense to replace the date placeholder at
process creation, it has to be replaced at every start of the process.

On process creation only the static placeholders (such as process ID)
are replaced. Dynamic placeholders (so far only "date") are not
replaced. On process start, a callback has been introduced that gives
the chance to change the command line. This is the point where
the restreamer replaces the date placeholders.

ffmpeg/ffmpeg.go

@@ -32,9 +32,10 @@ type ProcessConfig struct {
 	Reconnect      bool
 	ReconnectDelay time.Duration
 	StaleTimeout   time.Duration
-	Command        []string
+	Args           []string
 	Parser         process.Parser
 	Logger         log.Logger
+	OnArgs         func([]string) []string
 	OnExit         func()
 	OnStart        func()
 	OnStateChange  func(from, to string)
@@ -113,12 +114,13 @@ func New(config Config) (FFmpeg, error) {
 func (f *ffmpeg) New(config ProcessConfig) (process.Process, error) {
 	ffmpeg, err := process.New(process.Config{
 		Binary:         f.binary,
-		Args:           config.Command,
+		Args:           config.Args,
 		Reconnect:      config.Reconnect,
 		ReconnectDelay: config.ReconnectDelay,
 		StaleTimeout:   config.StaleTimeout,
 		Parser:         config.Parser,
 		Logger:         config.Logger,
+		OnArgs:         config.OnArgs,
 		OnStart:        config.OnStart,
 		OnExit:         config.OnExit,
 		OnStateChange: func(from, to string) {