Use output.write instead of output.push, fixes #14

[goto-bus-stop]

In #14, something like the below happens:

• bytes are .piped to output
• output has to buffer at some point if it cannot drain fast enough
• the replacement ends, with some bytes still being in output's buffer
• static-module continues to push things to output, but push bypasses its buffer
• output pushes things from its buffer, out of order

By doing output.write instead, we always go through output's buffer when necessary, i.e. take the same path as .pipe().

Not quite sure how to test this just yet…!