Generate a single feature-icons.json and feature-icons.css.
In the JSON, features with a line icon are indicated by the
presence of a `line` property. PresetIcon looks for this to
determine whether or not to add the `preset-icon-line` class.
New line icons may be added by editing line-icons.json.
This directory will contain the built sources (iD.js, iD.css)
and images. On a release branch, we will check in the generated
files. For the osm.us/master deploy, it should run make and use
dist/ as the root.
Paths in css are relative, e.g. url(img/foo.png).
