Add a target type for indexed luigi.LocalTarget

[arteymix]

This would apply to indexed BAMs and VCFs and possibly other tabular format with tabix. This would basically turn the folowwing:

class TaskThatProducesIndexedOutput(luigi.Task):
    def output(self):
        return luigi.LocalTarget('test.vcf.gz'), luigi.LocalTarget('test.vcf.tbi.gz')

Into:

class TaskThatProducesIndexedOutput(luigi.Task):
    def output(self):
        return bioluigi.targets.IndexedLocalTarget('test.vcf.gz')

It would be considered complete if both resulting local targets exist.

[arteymix]

This would be used in the future GATK module. It can also be readily used in the chromium-pipeline we already have to simplify some of our output/input logics.