Allow for stripping of base path for ignores

[toddself]

This fixes an issue where browserify-istanbul won't run if one of your parent directories is named node_modules. We want to ignore children directories name node_modules but not parent directories from the root directory in which we're running our tests.

This is required by our setup, as we're using the same development setup w/r/t to directories as the dat project: https://github.com/datproject/dat/blob/master/CONTRIBUTING.md#developing-inside-a-node_modules-folder

[toddself]

hi! would love to get this merged in, or to figure out a better way to get this functionality to work. open to any suggestion!

[alexindigo]

Hey, sorry I missed it somehow. Are you still around to go over this?

Thank you.

[toddself]

@alexindigo sorry i missed this entirely too. I'm happy to go over this!