[proposal] allow for install with subset of apps

[vsoch]

We can currently install the entire contents of a recipe.scif (the "publish one container" use case)

scif install recipe.scif

or break our recipe into more modular applications (the cluster "make your own container" example);

scif install samtools.scif
scif install bowtie.scif
scif install tophat.scif

There is a third use case where we have the first single file, but we only want to install a subset of applications. We should be able to do:

scif install recipe.scif samtools bowtie

to install all except tophat, for example.