enh: .jpmignore should be a cli or environment option

[gregglind]

Currently, '.jpmignore' in lib/ignore.js is hard coded. It would be nice if that could be expanded to a cli option, for this reason:

1. I want different .jpmignore during tests, to allow for more node_modules and coverage directories, etc.

[kumar303]

Sure, a command line switch or env var to set a custom ignore file sounds useful