Add exponential moving average to measure habit strength

[lashoun]

Description

This solves #84 and also changes the formula of the daily average number of cards so that it also uses an EMA, which should be more comparable to current reviews.

Checklist:

• I've read and understood the contribution guidelines
• I've tested my changes against at least one of the following Anki builds:
  • Latest standard Anki 2.1 binary build
  • Latest alternative Anki 2.1 binary build
• I've tested my changes on at least one of the following platforms:
  • Linux, version:
  • Windows, version 2.1.54:
  • macOS, version:
• My changes potentially affect non-desktop platforms, of which I've tested:
  • AnkiMobile, version:
  • AnkiDroid, version:
  • AnkiWeb