add :force_load option into configuration of `app.vendor_project'

There are 3rd-party library which would trigger "duplicate symbol" error at compile time.
It will be useful in such libraries when specify false in :force_load option.

When specify true in this option (by default), it behaves in the same manner as before.

Usage:
    app.vendor_project('vendor/OAuth2Client',
      :static,
      :force_load => false,
      :headers_dir => 'Sources/OAuth2Client'
      ....