Add global options to Jakefile

[luvies]

Could you add a global options object that allows setting defaults for various options? For example, the exec function current defaults to printStdout and printStderr to false, but with the global options you could do something like the following:

jake.options.exec.printStdout = true;
jake.options.exec.printStderr = true;

And this would mean that all future calls to jake.exec would have both options true by default.

[imhaage]

It would be great to have this feature. If you don't have time and if you agree I could try to implement it.

[mde]

I'd be happy to merge a PR that implements this!