Add a build step to generate extended metrics that additionally contain glyph widths

Summary:
The ability to use pre-determined character widths will benefit alternative
layout engines such as gagern's canvas layout engine.  I would also like to
experiment would using CSS transforms to absolutely position each glyph.  This
diff adds a new make rule, make extended_metrics, which generates metrics that
also containing glyph widths.

Test Plan:
- run `make extended_metrics`
- verify that fontMetricsData.js contains entries with 5 numbers instead of 4

Reviewers: emily alpert

metrics/format_json.py

@@ -3,6 +3,12 @@
 import sys
 import json
 
+props = ['depth', 'height', 'italic', 'skew']
+
+if len(sys.argv) > 1:
+    if sys.argv[1] == '--width':
+        props.append('width')
+
 data = json.load(sys.stdin)
 sep = "module.exports = {\n"
 for font in sorted(data):
@@ -11,10 +17,8 @@ for font in sorted(data):
     for glyph in sorted(data[font], key=int):
         sys.stdout.write(sep + json.dumps(glyph) + ": ")
         
-        values = [data[font][glyph][key] for key in
-            ['depth', 'height', 'italic', 'skew']]
-            
-        values = [value if value != 0.0 else 0 for value in values]
+        values = [value if value != 0.0 else 0 for value in 
+                  [data[font][glyph][key] for key in props]]
             
         sys.stdout.write(json.dumps(values))
         sep = ",\n  "