pre-fetched thumbnails in locate_areas to improve speed

[jkeuskamp]

@leeper I have cloned your excellent package as I am planning to make some minor modifications for my specific use-case. It may be that you wish to incorporate some of them in you package.

I have for example added the option to use pre-fetched thumbnails for use in the locate_areas function to improve loading speeds. The function now has an extra option that can be used to inducate a directory containing thumbnails generated with the make_thumbnails function.

I have tested this and it seems to work fine. The dramatically decreased loading time are helpful if one has to process a large number or files. Would you like me to do a pull-request or would you rather not have this possibility incorporated?

[leeper]

Sure. Open a PR.