Modify copy function to compress payload

[vladimirvivien]

Currently the copy command copy the specified file(s) to the local machine where crashd is running. As a crashd script writer, I should be able to have crashd copy and compress the file as it is being copied to save an extra step.

For instance:

copy(path="path/to/copy", format="gz")

The example above would copy the file from the remote machine and stream and gzipped it as it is downloaded.

[vladimirvivien]

First attempt at implementing this was using the -C flag for SCP command. However, that flag compresses the network communication between client/server. The final downloaded file is uncompressed.

Workarounds:

1. Continue to use SCP and compress file after it is copied locally (slow because file has to be downloaded first then compressed).
2. Update capture(...) and change it to compress as file is stream from stdout from server (slow, since content has to stream over stdout, but compression can happen inline as file is streamed. So may be better then option 1).
3. The best would be to do a Go implementation of SCP and compress io stream as it it received from server.