Respect VCS ignored files and folders

[ericcornelissen]

This seems tangentially related to #137. However, I would like to suggest that the Fossa CLI should automatically respect files and folders already ignored by VCS.

I'm requesting this because I have some projects where project manifests end up in a temporary directory that's ignored by my .gitignore, and the Fossa CLI recognizes this and analyzes it. I don't think it's ever the case Fossa should analyze things ignored by VCS, but if there is it could be made optional somehow.

I know I can use the targets.exclude option to achieve this. However, this is not an ideal solution as it means I have to keep my ignores/excludes in sync.