feat: add optional support to respect the git ignorefile

[Kenneth-Sills]

This is a simple flag configuration that controls whether the git commands we run internally respect .gitignore (or whatever core.excludesFile is set to), allowing downstream users to specify more broad assets settings that are then restricted by the local ignore file.

This is particularly useful when making shared configurations that, in my particular case, may need to perform last-minute file updates via the postversion NPM lifecycle script that must be committed to the project repository (updating the version in manifest.json for a browser extension).

Based on maintainer opinion in #55 / #357 to avoid increasing maintenance burden and a desire to keep backwards compatibility, I've made sure this is an optional configuration option that defaults to being disabled.

Closes #345 Closes #347

[pysiekytel]

@travi is there any way to get this reviewed and merged? That would be helpful