Add diagram indexing support for all formats

[DRKV333]

Currently, the only format that properly supports the idx parameter is base64. All other formats only let you select which image you want to export from the first multi-image diagram.

So if you have:

@startuml
class a
newpage
class b
@enduml

And POST it to /svg/1, you do get a class b in the output.

But if you have:

@startuml
class a
@enduml
@startuml
class b
@enduml

POSTing that to /svg/1 results in a class a. But POSTing it to /base64/1 does result in class b correctly.

This is unfortunate, since it makes exporting diagrams that use @startdef in svg completely impossible, since the @startdef always has to come first. To fix the issue, I have partially copied some of this code: https://github.com/plantuml/plantuml/blob/21461b83fc96a733f6879f18a7f0687d6e2ce456/src/net/sourceforge/plantuml/SourceStringReader.java#L154-L177 The solution is a little bit different from the base64 case, returning a 400 result, if the requested image index doesn't exist. Base64 returns 200 with an empty image, which I didn't change, but I think a 400 is more appropriate here.

[arnaudroques]

Thanks for your contribution!