Rename instances of "reply" to "replay"

This looks to have been a typo. Limited to file-based playback only. The
word "replay" makes more sense here, and is consistent with TCP playback.
Merge pull request #40 from alphagov/rename_reply_to_replay

Rename instances of "reply" to "replay"
Merge pull request #41 from alphagov/readme_file_based_timings

Document time delay in file-based replay
Update README.md
Allow injection of additional headers in replay

Similar to curl(1) and siege(1). Can be specified multiple times. Will
overwrite headers of the same key name in the original request. Arguments
without a colon will be discarded and an argument ending in a colon will
result in an "empty" header with no value.

No tests, yet.