Downsampling support

[grendon]

Hi,

The size of the illumina short-reads files continue to grow as the sequencing costs continue to fall. Running FastQC on the entire input file is compute-intensive. Performing the downsampling step before running FastQC is also compute-intensive. It would be great if FastQC could grab a subset of the input by downsampling randomly and then run the analyses on the smaller set rather than on the entire input file.