Support ktfmt to format Kotlin

[dbarnett]

Formatter tool
ktfmt: https://facebookincubator.github.io/ktfmt/

Filetype(s)
kotlin

[dbarnett]

Note there's also #127 to support an alternative ktlint formatter. This ktfmt tool (which is based on google-java-format engine) claims to produce more consistent formatting and handle some corner cases better: https://github.com/facebookincubator/ktfmt#ktfmt-vs-ktlint-vs-intellij.