Fix image generation when a name is defined on startuml

[VitorVRS]

This address this issue: https://github.com/weirongxu/plantuml-previewer.vim/issues/23

According to this post, the output filename will be the as the startuml value, when present.

The solution is use the pipe parameter from plantuml.jar to create the output file, instead of the default behavior.

This is a workaround, feel free reject and do a proper fix.

[weirongxu]

Thanks for the explanation and PR

For Linux this is a solution, but it's not perfect, because it does not support Windows, and we have to consider supporting new_page #19, maybe we should consider refactoring the code to read photos

new_page

[VitorVRS]

but it's not perfect

Totally agree.

I just opened the PR in order to make this hotfix documented to any other linux user with the same issue.

There is a room for improvement and refactoring but I don't want to get into, tbh.