Load classpath lazily and add -jrt option

This brings total time needed to decompile Minecraft 1.18.2 from
~38 seconds, to ~25 seconds on my machine. Decomplication can be quite
easily performed with only 2GB of memory allocated to the JVM, when
previously 4+GB was required.

Add -jrt option to enable decompiling with classes from another JVM

Author: zml2008