support of encoding setting/detecting for text file

[ChetWang]

Some jars have text config files, such as xml/yml/properties/json files. jd-gui use system encoding while opening those files. But some times, another chaset is needed. For example, most jars are compiled、packaged with charset 'UTF-8', but user system encoding may be another charset, it is very common for Asian users.

[liudonghua123]

I set an environment variable JAVA_TOOL_OPTIONS=-Dfile.encoding=UTF-8 -Dsun.jnu.encoding=UTF-8 to overwrite the default system encoding like gbk or others.

You could also pass the option in java command when execute the jar. For example, java -Dfile.encoding=UTF-8 -Dsun.jnu.encoding=UTF-8 -jar /path/to/jd-gui-1.6.6.jar /optional/path/to/jar/or/class.

see https://stackoverflow.com/questions/361975/setting-the-default-java-character-encoding.