Implement force option to bypass JAVA_HOME check

[explunit]

Allow running the task even when JAVA_HOME environment variable is not set. jsdoc itself does not appear to require this in all cases.

[iangilman]

+1

[krampstudio]

For your information, the grunt cli --force option already work:

grunt jsdoc --force

[explunit]

Thanks, this change simplifies things for me.

I think the message is still somewhat misleading, as JAVA_HOME doesn't need to be set in order for jsdoc to run, and simply installing Java does not set this variable. But in any case, it works now.