Add configuration for output format and prefixes

[DannyBen]

This PR comes to address the need (issue #1) for removing backticks and [stdout]/[stderr] markers from the output in some cases, like when using with HipChat.

It adds these three optional configuration parameters:

  config.handlers.cmd.output_format = "/code %s"   # example for nice HipChat output
  config.handlers.cmd.stdout_prefix = ""
  config.handlers.cmd.stderr_prefix = "ERROR: "

I have left the defaults as they were, and added documentation in the README.

[dfinninger]

Actually hadn't looked at this before writing #8. I'll take a look and see we can work this in.

[dfinninger]

merged with 6aa45ca