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.
- An argument without a value (`Foo:`) will be set as empty.
- Leading and trailing whitespace will be stripped from the key and value,
  otherwise it's difficult to determine where the key or value begin/end.

No tests, yet.