[ADD] File merge function

[kang0921ok]

I added a file merge function.

I used BufferWriter.

It is sovled #255

AS-IS
a.js b.js c.js -o min.js
--> c.js == min.js

T0-BE
a.js b.js c.js -o min.js
--> a.js + b.js + c.js == min.js

Thanks

[tml]

Thanks for the contribution!

There are a handful of whitespace-only changes in this commit (3 changes to indentation level and a missing file-terminating newline); could you update the PR to remove those? If the whitespace needs to change for logical/consistency reasons, I'd prefer keeping those in distinct whitespace-only commits.

[kang0921ok]

Okay, I remove whitespace.
Check Plz man~

Thank you for confirming my PR.

[kang0921ok]

@tml But This is not concerned That Same variable name. is It ok?