further PlantUML integration

[McFoggy]

It would be good to start thinking how to integrate into gitbucket rendering mechanism so that inline PlantUML code or references to PlantUML files could be rendered as an image.

What I am thinking of:

1. Render PlantUML code in Markdown (README, wikis, ...)

For example embedding the following in markdown

    ```plantuml
    Bob->Alice : hello Alice, I'am Bob
    Alice-->Bob : hello Bob
    ```

2. Render references to PlantUML files

this is already possible in mardown using images links and the proxy available on PlantUML site (see here).
The drawback of that is that it requires access to the internet which is not the case in all the deployments of gitbucket.
Perhaps a similar feature could be implemented to act the same.

[McFoggy]

I think this could be easily tackled once gitbucket/gitbucket#1307 & more precisely gitbucket/gitbucket#1323 will be done.

[nus]

1. Render PlantUML code in Markdown (README, wikis, ...)

Sounds good.

[nus]

2. Render references to PlantUML files

@McFoggy

Now, it is possible to download a PlantUML image from gitbucket on 7cd3b41 .