Add default value when SYSL_PLANTUML is unset

[ChloePlanet]

Current Sysl Version

v0.123.0

Use-cases

export SYSL_PLANTUML=http://www.plantuml.com/plantuml is required to generate diagram at present. Adding a default value to it if SYSL_PLANTUML is unset.

[anzdaddy]

Changing the default to a public service is dangerous, since it will result in accidental leakage of work product from organisations using Sysl internally. A better solution is to keep the default but provide helpful instructions in the error output if localhost:8080 doesn't work.