Add symlink for src/config directory

[mike-potter]

Added steps in scaffold.js to add the symlink from drupal docroot/config to src/config.

[grayside]

1. Basic test coverage is needed. This will require creating examples/src/config/.gitkeep
2. As written, this seems like it will throw warnings if not errors for any project that upgrades to a version of GDT with this but does not have a src/config directory. While we would like D8 projects to use this, many will not, and I doubt we'll ever see a D7 project use it.
3. This needs docs/

[grayside]

We do not need to do Drupal core version detection, we should instead make the presence of src/config optional.

[scottalan]

Brought this up in a meeting with @grayside. Wanted to link to this issue here: https://www.drupal.org/node/2899534

[scottalan]

Basic test coverage is needed. This will require creating examples/src/config/.gitkeep

@grayside or @mike-potter In Atmosphere we are using config_split. We opted to create our directory structure like this, src/config/default, where default holds our main site configuration. We then have configuration that only exists in specific environments in other directories. Do we think it makes sense to create src/config/default/.gitkeep to make it easier for projects that will need to use config sync? I ask this purely from an organizational standpoint.

[grayside]

There are two basic approaches we can take as far as symlinks: we can link to src/config from build/html/config, or we can link to src/config/* from build/html/config/*. For the latter, I mean the approach taken with symlinking profiles as opposed to the approach of symlinking modules (which uses build/html/modules/custom --> ../../../../src/modules.

I do not think gdt needs to present support for Config Split, but if having a config/default directory seems reasonable in the absence of config split I don't object.

[grayside]

Superceded by #330