Added lodash as a dependency.
Improved the file picker button directive and started using it.

    * The file picker button can now call a function in the
      calling scope whenever the user selects new files.
    * <ddt-font-card-add> now uses this directive to pick
      files.
Added a bottom margin to font cards.
Fleshed out the font family collection.

    * Added constants for events broadcast by the collection.
    * Broadcast an event every time a new item is added to the
      collection.
    * Exposed a new method to generate a placeholder name for
      new families.
    * Exposed a method that returns the array of families.
FontFamily.addFontFromFile now uses opentype.js to load fonts.
Fleshed out <ddt-font-card-add>.

<ddt-font-card-add> is now able to create font families, add
fonts to them, and add the families to the global font collection.
<ddt-font-cards-view> now renders grids of word cards correctly.