Allow for arrays as templateAttributes

Use case: We want to define a list of custom fonts per target within the generated Info.plist.

This is our template (simplified):

targetTemplates:
  App:
    info:
      UIAppFonts: ${fonts}

This is the target (automatically generated, based on a tenant configuration file and folder content):

targets:
  Tenant1:
    templates:
      - App
    templateAttributes:
      fonts:
        - MyFont-Italic.ttf
        - MyFontBlack.ttf

So, I would want the resulting Info.plist to contain:

<key>UIAppFonts</key>
<array>
  <string>MyFont-Italic.ttf</string>
  <string>MyFont Black.ttf</string>
</array>

However, in the current state of XcodeGen, the result is:

<key>UIAppFonts</key>
<string>${fonts}</string>

Could this be a useful addition?

Start by tracing how targetTemplates and templateAttributes are expanded into Info.plist values. Confirm the change by generating a project from the YAML examples and checking that ${fonts} becomes an array of strings in the resulting Info.plist.