Fixes #6902

Fixes #6556

fixes #5867

fixes #1033

This commit adds the --user-data-dir argument which allows a custom data dir
for Chromium's user data. This is useful when running as root on Linux since
the root user does not have read or write permissions for the ~/.config/Code
dir.

Part of #3068

This enables git integrations by waiting until the window is closed before
terminating the process. To set code as the default git editor run:

    git config --global core.editor "code --wait"

Fixes #744

Fixes #3565