Improve zooming performance and UX

[kimikage]

Fixes #29
preview: https://kimikage.github.io/ProfileSVG.jl/previews/PR30/

This will be the last update in the v0.1 series.

[codecov-io]

Codecov Report

Merging #30 into master will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
+ Coverage   89.01%   89.13%   +0.11%     
==========================================
  Files           2        2              
  Lines          91       92       +1     
==========================================
+ Hits           81       82       +1     
  Misses         10       10              

Impacted Files	Coverage Δ
src/svgwriter.jl	100.00% <100.00%> (ø)

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