Autoselect exec mode

[skuro]

Summary

When no explicit preference is given for plantuml-exec-mode, plantuml-mode tries first executable then jar. If both fail, the user is asked whether to use server mode against plantuml-server-url.

Background

As outlined in #112 when running plantuml-mode preview functionalities right after install, the default exec mode is server which sends data to a possibly untrusted web application. This should be done silently only if the user explicitly requested server exec mode.