Add image sizing with width and height config fields.

[luixal]

Adds two new fields width and height that can be used to set the size of the image.

This example:

type: custom:media-source-image-card
image: >-
  https://t4.ftcdn.net/jpg/01/43/23/83/360_F_143238306_lh0ap42wgot36y44WybfQpvsJB5A1CHc.jpg
width: 300px
height: 200px

would render as this:

You can use any value that's css valid (100px, 100%, 10rem, 10em, etc.). And you may use card_mod or tweak your theme for it to look pretty :P

Closes #17