Refactor application structure and initialize core components

I decided to rewrite the program taking into account the experience gained.

internal/ffmpeg/ffplay.go

@@ -0,0 +1,19 @@
+package ffmpeg
+
+type FFplayContract interface {
+	SetPath(path string)
+}
+
+type ffplay struct {
+	path string
+}
+
+func newFFplay(path string) FFplayContract {
+	return &ffplay{
+		path: path,
+	}
+}
+
+func (f *ffplay) SetPath(path string) {
+	f.path = path
+}