add csv generator command

[jaffee]

I'm thinking we can use this to generate some benchmark data for ES.

Commands should be something like:

pi csvgen -f runs.csv --rows=100 --runs-max=2000 --n-max=10000 --cols=5000000
pi csvgen -f arra.csv --rows=100 --runs-max=3000 --runs-min=500 --n-max=3000 --n-min=0 --cols=5000000
pi csvgen -f bitm.csv --rows=10 --runs-max=20000 --runs-min=2000 --n-min=4100 --n-max=20000 --cols=5000000

[jaffee]

@alanbernstein thanks for letting me steal and misuse your code - any review would be appreciated

there is definitely some weirdness with validating the arguments and deciding how to handle when the generated N is less than the generated num runs, etc.