Suport cygwin when plantuml-exec-mode is 'jar

[takushi1969]

Convert plantuml-jar-path in cygwin format to windows format.

ex. ~/pllantuml.jar -> /C:\cygwin64\home\takus\plantuml.jar

Because in cygwin, java is used the one of windows native app and it can only accept jar-path in windows format.