Allow user defined blacklisting of libraries

[mboes]

As described in #80, sparkle package creates self contained .jars with all .so's embedded in it. But for testing/debugging/temp workarounds, it may be useful to force the usage of system versions of select libraries instead. Further, target systems may sometimes contain optimized vendor versions of performance sensitive codes (e.g. BLAS) that are not necessarily available in the build environment.

We should add a flag to the sparkle command-line tool make it easy to "blacklist" libraries from being included in the closure.