Add aria-label parameter

[fernandezcuesta]

Reporters like axe complain about treeselect component's input missing a11y parameters.
This adds an additional (optional) parameter to set the aria-label value for the input tag.

[codecov]

Codecov Report

Merging #294 into master will decrease coverage by 0.19%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #294     +/-   ##
=========================================
- Coverage   97.88%   97.68%   -0.2%     
=========================================
  Files          27       27             
  Lines        1038     1038             
  Branches      104      104             
=========================================
- Hits         1016     1014      -2     
- Misses          7        9      +2     
  Partials       15       15

Impacted Files	Coverage Δ
src/components/Input.vue	98.97% <ø> (ø)	⬆️
src/mixins/treeselectMixin.js	100% <ø> (ø)	⬆️
src/components/MenuPortal.vue	93.65% <0%> (-3.18%)	⬇️

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[tomearly]

Is this something that can be added still?

[iampawan31]

Any idea when this PR will be merged? need this feature on the plugin.