umlet_binary_path: allow to specify command and args

[niziak]

I want to specify what to call to run umlet. Under Debian distro umlet.sh is not avail by default, and direct .jar execution is not possible because kernel limitation to support binmisc_fmt.

So I want to specify umlet_binary_path like: umlet_binary_path = "java -jar /usr/share/java/umlet.jar"

But this variable is used as executable name, resulting with error:

[Errno 2] No such file or directory: 'java -jar /usr/share/java/umlet.jar'

See how it is handled in plantuml.py