Fix handling of included feature files

[justvanrossum]

This fixes #149.

However: the test font (ExternalFeatureFile.glyphs) still does not work with Fontra or fontmake, because it causes the expanded features to have a languagesystem statement that is not at the top of the file.

This file was modeled after a real project, and I'm not sure if this is a construct that Glyphs handles fine on export and only fontc and fontmake complain about, or whether the test file ended up being atypical. Cc @faebiou, @RafalBuchner.

[justvanrossum]

FWIW it's about this fragment in the .glyphs file:

featurePrefixes = (
{
code = "include(ExternalFeatureFile.fea);
";
name = include;
},
{
automatic = 1;
code = "languagesystem DFLT dflt;
";
name = Languagesystems;
}
);

The included file comes first bit (the file contains a languagesystem statement), and after the included code we'll see the second languagesystem DFLT dflt (from the second part of the snippet), which causes the errors in fontmake and fontc.

[justvanrossum]

@khaledhosny, do you happen to know any Glyphs-based projects that are built with fontmake and use an external .fea file?

[khaledhosny]

Inter seems to use external feature files: https://github.com/rsms/inter/tree/master/src

[justvanrossum]

I checked with the Inter sources, and the languagesystem declaration in the .glyphs data comes before the include section, and therefore works out of the box with this PR. I've adjusted the test .glyphs file here to follow that pattern.