Fix string interpolation for plist config

In the plist section of the ios `config.json` for plugins, strings are
not correctly interpolated. For example,

```json
{
  "plist": {
    "foo": "bar${baz.qux}"
  }
}
```

Will now set `foo` to "bar" + the value of `baz.qux` from the manifest
addons section.
Fix plugin file copying instead of reference

Files from `code` and `arccode` in plugin config are now _referenced_
from the xcode project instead of being copied in. In the case of the
code section, we fix an issue with absolute paths.
Add new plugin header/lib search path support

The folders `lib` and `include` are now added as a library search path
and header search path, respectively, to the XCode project. The paths
are made relative from the project folder to the plugin folders.
Additionally, the `libs` keyword in `config.json` can be used to specify
a list of static libraries to link against. This is now preferred over
putting them in the `frameworks` list which causes the libraries to be
copied into the project.

The node-xcode project was forked to make this possible. A github
tarball link at the appropriate ref is now listed in the package.json
file instead of the npm package.