allow changing image derivative sizes via a config file

[thatbudakguy]

Is your feature request related to a problem? Please describe.
the strategy for automatically generating image derivatives implemented in #117 hard-codes a particular set of image sizes (widths) to generate (500px, 800px, etc) so that the correct size can be used to fit the device. these sizes could be defined in a configuration file so that they're easier to edit later on. for earlier discussion, see #117 (comment).

Describe the solution you'd like

• a data file is created in the theme folder or project root that defines the image sizes to generate for each image
• image derivatives are automatically generated using these sizes when someone embeds an image using the figure shortcode on a markdown page

Additional context
see #117 for current image resizing implementation and #115 for example of reading values from a data file