feat: convert to plain text

model.go

@@ -9,6 +9,11 @@ type Lyrics struct {
 const USAGE = `Usage: lyc-cli [command] [options]
 Commands:
   sync	Synchronize timeline of two lyrics files
+  convert	Convert lyrics file to another format
   help	Show help`
 
 const SYNC_USAGE = `Usage: lyc-cli sync <source> <target>`
+const CONVERT_USAGE = `Usage: lyc-cli convert <source> <target>
+Note:
+Target format is determined by file extension. Supported formats:
+  .txt	Plain text format(No meta/timeline tags)`