Support PlantUML within Markdown

[paulvi]

There are several places where PlantUML within markdown is already supported

e.g. https://docs.gitlab.com/ee/administration/integration/plantuml.html

Code Example

```puml
    Mardown -> PlnatUML : hello
```

Please support within this plugin

Currently it does not

[paulvi]

Currently requires @startuml and @enduml

[hallvard]

OK, so the proposal is to support a wider set of prefix and suffix pairs.

[paulvi]

Yes, prefix ```puml is already used on https://docs.gitlab.com/ee/administration/integration/plantuml.html

But the rest of ```puml line may have some additional parameters, so actual PlantUML will start on next line.

Within ``puml block can be other type of diagram, not UML, e.g.@startsaltand@endsaltpair, or@startdotand@enddotpair, so@startumland@enduml` pair becomes default.

For example a file with extension .puml and single line

Bob->Alice : hello

is also valid PlantUML document/diagram, isn't it?

[hallvard]

It's easier to support special prefixes/suffixes when they're tied to a specific editor or file extension, like md or puml. So if it's OK with you, I'd rather add this as md- and puml-specific rules.

[paulvi]

As you prefer