Optionally make intermediate filler groups

This commit adds a new option `createIntermediateGroups` that defaults
to false. When it is false, the behavior of XcodeGen is the same as
before. When it is true, we make intermediate groups recursively until
we reach the basePath. In practice that means if you've chosen
`Platform/PINFoundation/Sources` as one of your sourcePaths, you get a
top-level group of `Platform` and under that `PINFoundation` and under
that `Sources`. This is instead of the default behavior of just making
`Sources` a top-level group (which is confusing when your directory is
called `Sources` for example).

docs/ProjectSpec.md

@@ -63,6 +63,7 @@ Note that target names can also be changed by adding a `name` property to a targ
 
 ### Options
 - ⚪️ **carthageBuildPath**: `String` - The path to the carthage build directory. Defaults to `Carthage/Build`. This is used when specifying target carthage dependencies
+- ⚪️ **createIntermediateGroups**: `String` - If this is specified and set to `true`, then intermediate groups will be created for every path component between the folder containing the source and the base path. For example, when enabled if a source path is specified as `Vendor/Foo/Hello.swift`, the group `Vendor` will created as a parent of the `Foo` group.
 - ⚪️ **bundleIdPrefix**: `String` - If this is specified then any target that doesn't have an `PRODUCT_BUNDLE_IDENTIFIER` (via all levels of build settings) will get an autogenerated one by combining `bundleIdPrefix` and the target name: `bundleIdPrefix.name`. The target name will be stripped of all characters that aren't alphanumerics, hyphens, or periods. Underscores will be replace with hyphens.
 - ⚪️ **settingPresets**: `String` - This controls the settings that are automatically applied to the project and its targets. These are the same build settings that Xcode would add when creating a new project. Project settings are applied by config type. Target settings are applied by the product type and platform. By default this is set to `all`
 	- `all`: project and target settings