SPK project should better handle pre-existing git ignore files

**As a:**
Developer

**I want:**
If a .gitignore file exists `spk project init` should not create/overwrite but instead append to it (and deduce)

From https://github.com/microsoft/bedrock/issues/1009 cc @jmspring 

**So that:**
My `.gitignore` is not overwritten

**Does this require updates to documentation?:**
YES 