1.3.0: Add a prop to disable automatic font size recalculation

[dumptyd]

This PR implements the following changes:

Library

• Add auto-adjust-text-size boolean prop to enable/disable automatic font size recalculation. (resolves Add a prop to disable font size recalculation #29)
• Minor refactoring.
• Upgrade to Vue 2.6.11.
• Upgrad some vulnerable dev dependencies.

Documentation

• Add documentation for auto-adjust-text-size.

Demo

• Add an option to demonstrate auto-adjust-text-size on demo page.

Implementation

Instead of simply returning from the font recalc function, I went with a slightly different approach that takes care of conditionally registering/removing resize listener for the chart based on the prop's value.

[codecov]

Codecov Report

Merging #30 into master will not change coverage by %.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #30   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         5           
  Lines          115       121    +6     
  Branches        24        28    +4     
=========================================
+ Hits           115       121    +6     

Impacted Files	Coverage Δ
src/components/Donut.vue	100.00% <100.00%> (ø)

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