v0.05 - abundance filter

• Added a new abundance filter that searches for kmer "valleys"
• Enable this filter by specifying --mindepth 0 when using fastq files
• idea from Alex Jironkin and Anthony Underwood, but I created a version that is unfortunately not as fast but does not require any additional dependencies.
  • https://bitsandbugs.org/2016/07/29/mash-and-khmer-abundance/