Further improvements to file size estimate #211

https://github.com/sindresorhus/Gifski/pull/205 made the estimate much more accurate, but we can still improve it:

• We currently add 10% to ensure the estimate is always more than the actual size. The percentage could be more dynamic: https://github.com/sindresorhus/Gifski/pull/205#issuecomment-709971974
• Show a range for the naive estimate: https://github.com/sindresorhus/Gifski/issues/130
• For the naive estimate:

  Once you get any better estimate, remember the ratio between the naive and the better estimate. Apply that ratio to later naive estimates. This will give you decent estimates real time for changing parameters.

• Use machine learning to improve the naive estimate?
• Refactor the code to use a different method for file size estimation: https://github.com/sindresorhus/Gifski/blob/e6c97bc06b036b274563de5a2638fdb663931649/Gifski/Gifski.swift#L214

Other ideas welcome.