Editor prompt type enhancement - Allow specifying temp file extension to get syntax highlighting

[akaleeroy]

If the temp files had an extension editors could immediately provide syntax highlighting, linting etc.

node-external-editor config provides

postfix (string) Optional A postfix for the file name. Useful if you want to provide an extension.

passed on from the tmp package.

Maybe type, name, message [, extension, default, filter, validate].
Or fully type, name, message [, tmpConfig, default, filter, validate] ?

[SBoudrias]

Good idea! If you're willing to send a PR, I'll help to review, merge and release.