Add an imgbasename option to the UML directive

[fconil]

Hi,
I wished to be able to specify the names of the generated diagrams with an imgbasename option in the uml directive. I wanted to be able to store the generated diagrams with a meaningful name just in case.

I saw that you won't rewrite the file if it has the same name. In my case, I want to overwrite the file, so I have added a global option plantuml_overwrite_output. It's True by default and I realize that it may be wrong as this will change the behaviour of your extension.
Regards

[yuja]

Well, the hashed filename is the cache key to avoid running plantuml unless
necessary. Running plantuml is painfully slow.

Perhaps, you can instead copy (or symlink) a generated file to the imgbasename
location if specified.