Jbusecke ffmpeg integration

[jbusecke]

This PR automates the movie/gif creation behind the scenes. This enables xmovie to run with just 2 lines of code, making it very beginner friendly.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@841851a). Click here to learn what that means.
The diff coverage is 71.65%.

@@            Coverage Diff            @@
##             master       #3   +/-   ##
=========================================
  Coverage          ?   67.96%           
=========================================
  Files             ?        2           
  Lines             ?      256           
  Branches          ?       33           
=========================================
  Hits              ?      174           
  Misses            ?       67           
  Partials          ?       15

Impacted Files	Coverage Δ
xmovie/presets.py	57.01% <50%> (ø)
xmovie/core.py	76.76% <72.72%> (ø)

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