add server-pico as option for exec-mode

[timcharper]

server-pico is a minimal plantuml web server that does not support the /language endpoint To cope with this, we grab language settings from the jar itself, but then use server-pico for rendering

[timcharper]

This may not be the best way to support pico support. Perhaps it'd be ideal to push for support for the /language endpoint for it? There's no good reason it couldn't support that endpoint, since the JAR clearly has the ability to generate the text it would be a matter of adding support for it.

Also, it might be nice to have an option to automatically start the pico server inside of emacs.