Add support for include in project.conf

Description

Add support for (@) as include directive.

Changes proposed in this merge request:

• Change ownership of Loader to Project. This is required because Project needs to load junctions.
• Make Loader constructor not know about targets. We know about targets only after the project is loaded.
• Support include directives in project.conf and .bst files.
• Make Project loads project.conf in two pass.
  • First without includes to be used by junctions.
  • One with includes processed for all other elements.

This merge request, when approved, will close issue/bug:

Closes #331 (closed)