Allow processing of multiple directories

[kasparhuebener]

We use the script as a self-contained .jar version and process many directories:

java -jar excel-to-json.jar Dir1 --disable-watching
java -jar excel-to-json.jar Dir2 --disable-watching
java -jar excel-to-json.jar Dir3 --disable-watching

Because of the JVM load time, the process could be streamlined if an option allows pointing multiple directories (either directly or through e.g. textfile that contains all directories):

java -jar excel-to-json.jar Dir1, Dir2, Dir3 --disable-watching