Merge pull request #183 from googlefonts/YEXT

Adding [YEXT] Vertical Extension axis

Lib/axisregistry/data/y_vertical_extension.textproto

@@ -0,0 +1,16 @@
+# YEXT based on https://github.com/TypeTogether/Playwrite
+tag: "YEXT"
+display_name: "Vertical Extension"
+min_value: 0
+default_value: 0
+max_value: 100
+precision: 0
+fallback {
+  name: "Default"
+  value: 0
+}
+fallback_only: false
+description: "The axis extends glyphs in the Y dimension, such as the "
+  "Cap Height, Ascender and Descender lengths. This is a relative axis, "
+  "starting at 0% and going to the typeface's individual maximum "
+  "extent at 100%."