Read from Assets not Priv

Changes:

• Read SVG assets from the assets dir instead of priv. Thanks to @mhanberg
  This changes the default directory from priv/static/svg to assets/static/svg
  Note For migrating from v1.3.x you will need to remove the otp_app from the use PhoenixInlineSvg.Helpers
• Fix a bug where the app could crash if the SVG directory does not exist.