Provide a way to subset the vendored icon fonts

[yuhr]

Reading the documentation, from my naïve interpretation it seems like this library is embedding the entire font file into the bundle:

As with any custom font in Expo, you can preload icon fonts before rendering your app. The font object is available as a static property on the font component. So, in the case above, it is Ionicons.font, which evaluates to {ionicons: require('path/to/ionicons.ttf')}.

Typical users don't want to include the entire icon set, but just a few icons out of it. We can do subsetting manually and load it as a custom icon font, but it would be better if @expo/vector-icons can handle this scenario, maybe by providing a config plugin or somehow?