Customize plantuml server url

Hi,

Currently the plantuml server url is hardcoded, meaning we can only use the official plantuml server to generate diagrams:

typeorm-uml/src/builder/typeorm-uml.class.ts

Line 96 in 53d5b0e

return `http://www.plantuml.com/plantuml/${ format }/${ schema }`;

From a security point-of-view, sending a whole database structure to an external server so that it generates a diagram is not a good practice.

There are ways of hosting a plantuml server (eg. using this docker image)

Once hosted, it can be made available on any custom url - may it be accessible on the world wide web or just within an internal infrastructure.

In order for such a custom server to be supported by this library, it would be nice to be able to specify the plantuml server url as an optional flag.

--plantuml-url: string (default: http://www.plantuml.com/plantuml)

Feel free to take the pull request into consideration

Awesome! Thanks a lot, @raphael-leger!

Ok, published it as 1.6.0-1 via @next tag for testing.