Make final new line optional

[simonnilsson]

Why am I submitting this PR

A new line is always added at end of output files. Depending on auto-formatting behavior of editor e.t.c. this might not always be desirable.

Does it fix an existing ticket?

No

Checklist

• [x] only relevant code is changed (make a diff before you submit the PR)
• [x] tests are included and pass: yarn test (see details here)
• [x] documentation is changed or added