features

[lexelby]

Sorry it took me so long to create this pull request, @cyrilrey!

This adds a ton of fixes and features I found useful. My primary use case is automatically generating address labels and order slips for my Etsy shop orders. I also like to be able to print labels for my house :)

This PR adds:

• handle arbitrary template variables
  • automatically figures out the variables used by the Jinja template and creates input boxes for the user
• API support: a script can POST to /print with template variables to cause a label to be printed
• keep the last 10 printed labels as recent#.svg for easy reprints
• a few templates:
  • *banner*.svg -- big text printed in landscape
  • rickroll_qr.svg -- perfect for pranking folks ;)
• switch to rendering SVGs with Inkscape
  • so that we can support flowed text
  • also added caching of PNGs for templates on the main page because otherwise it would take forever to render all of them
• simplified scaling
  • set PRINT_WIDTH to the pixel width of the printer and the image is scaled to that width
• switch to brother QL-800

I had a heck of a time getting my POS-5890K working. I also couldn't find good continuous label tape for it, so I ultimately gave up on it. The Brother QL-800 has excellent support for continuous label tape. It also has a module very similar to python-escpos that allows for use of the printer without a kernel driver.

Obviously we would need to add support for python-escpos back in before merging this. Maybe some way to choose which module to use. I think in theory the QL-800 supports ESC/POS, but I didn't try using it with python-escpos...