Relax validation of reporter plan

Allow additional interpreters that aren't "required" by the reporter
Vcf reporter requires only minimal interpreters

We can now specify arbitrary "filterpreters" to be
used with the vcf reporter.  We don't need the specialized
vcf reporters that simply add another "required" interpreter.
Merge pull request #68 from apregier/relax-vcf-reporter

Relax vcf reporter