Add matplotlib to browser version

[donkirkby]

The browser version's turtle graphics support has all the same features as PyCharm, except create_image.

• [x] package matplotlib with deployment
• [x] implement create_image on canvas
• [x] try to avoid downloading matplotlib to browser when it's not imported by a script
• [x] clear all figures at start of run

[donkirkby]

Timing of Romania tutorial before I made matplotlib optional:

• GitHub after F5
  • Total 5.42 +/- 0.06s 9.5MB data transferred
• localhost after F5
  • Total 5.48 +/- 0.31s 41.1MB data transferred

Timing of Matplotlib intro:

• GitHub after F5
  • Total 11.13 +/- 0.97s 9.5MB data transferred
• localhost after F5
  • Total 10.05 +/- 0.13s 41.2MB data transferred

[donkirkby]

Timing of Romania tutorial after I made matplotlib optional:

• localhost after F5
  • Total 5.57 +/- 0.42s 19.8MB data transferred

Timing of Matplotlib intro:

• localhost after F5
  • Total 9.97 +/- 0.16s 41.2MB data transferred

So no significant change in timing, but less than half the data transfer. Maybe #503 will have more impact on timing.