support append file

[796F]

untested support for appending to file.

using it just fine, but might break dunno 🐙

[jprichardson]

Related: #23

With no tests, there's zero chance of this getting accepted.

I still have reservations regarding point 2 from #23:

I'm wondering if it'd be confusing that readFile, writeFile are used in the context of one JSON object (one JSON record), whereas appendFile would imply appending another JSON object, i.e. this JSON file would have multiple JSON objects (multiple JSON records). Would this be confusing? Is there a standard / mime-type of newline delimited JSON?

I've since learned there's ndjson and jsonnl standards. If you add tests, I'm still reluctant to accept this. Mainly because of the two different paradigms: writeFile is one JSON object, appendFile is many. What should readFile do then?