Improve bindown init to:

1. Add cache and install_dir to .bindown.yaml if they are set to non-default.

2. Add cache and install_dir directories to .gitignore.

Need to add an --install-dir param to init. It can use the global --cache option param for setting the cache dir. It
also needs a --no-gitignore param to prevent it.

The .gitignore file should be in the same directory as .bindown.yaml even if it is in a subdirectory of the git
repo.

The algorithm for .gitignore should be something like:

1. If git isn't available in PATH, abort
2. If .bindown.yaml isn't being created inside a git repo, abort
3. For each directory of cache and install_dir
   1. Use git check-ignore to see if the directory is already ignored. If so continue to next directory.
   2. Append the directory to .gitignore in the same directory as .bindown.yaml