minor optimization

[brownchow]

Hi oopsguy:

just some optimization

1. we don't need to check url variable in main.go, cause in download.go we have already deal with this situation.

I have tested on windows10, using git-bash

/d/golang/src/github.com/oopsguy/m3u8 (master)
$ ./m3u8.exe   -u http://127.0.0.1:1323/live/playlist.m3u8  -o  ""
[download  20.00%] http://127.0.0.1:1323/live/0/8.ts
[download  40.00%] http://127.0.0.1:1323/live/0/7.ts
[download  60.00%] http://127.0.0.1:1323/live/0/1.ts
[download  80.00%] http://127.0.0.1:1323/live/0/6.ts
[download 100.00%] http://127.0.0.1:1323/live/0/0.ts
[merge] ■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■ 100.00%
[output] D:\golang\src\github.com\oopsguy\m3u8\main.ts
Done!

2. update README.md