Make sure to draw the chart on a regular basis

[nakabonne]

Fixes: #75

Changes:

• Rather than calling the termdash API on every attack, made sure to buffer the value.
• Call the termdash API periodically (specified as redraw interval)

[codecov-io]

Codecov Report

Merging #77 into master will decrease coverage by 3.43%.
The diff coverage is 80.24%.

@@            Coverage Diff             @@
##           master      #77      +/-   ##
==========================================
- Coverage   75.71%   72.28%   -3.42%     
==========================================
  Files           8        8              
  Lines         535      541       +6     
==========================================
- Hits          405      391      -14     
- Misses         99      120      +21     
+ Partials       31       30       -1     

Impacted Files	Coverage Δ
attacker/metrics.go	100.00% <ø> (ø)
gui/keybinds.go	50.00% <0.00%> (-1.61%)	⬇️
gui/drawer.go	75.84% <82.86%> (-17.98%)	⬇️
attacker/attacker.go	68.63% <88.89%> (-2.38%)	⬇️
gui/gui.go	73.20% <100.00%> (+0.57%)	⬆️

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