Add Write

[mattn]

• add Write to embed to stream
• change command usage to handle stream

This PR change usage of command line.

embeding

$ zwfp -k payload text
$ cat text.txt | zwfp -k

extracting

$ zwfp text
$ cat embeded.txt | zwfp

please note that you will need to create new gif if you will merge this PR.

[vedhavyas]

@mattn sorry for the delay. Had a busy fortnight. Will review and get back 👍