hack for command list, support many data structure in tracks, support bamfile, bamfilelist, etc

git-svn-id: https://hedgehog.fhcrc.org/bioconductor/trunk/madman/Rpacks/ggbio@77653 bc3139a8-67e5-0310-9ffc-ced21a209358
support subseting tracks by '[' method

git-svn-id: https://hedgehog.fhcrc.org/bioconductor/trunk/madman/Rpacks/ggbio@77678 bc3139a8-67e5-0310-9ffc-ced21a209358