Adding slot for arrow icon

[avivday]

Will give the flexibility to change the arrow icon of the select box (many design teams ask for it) closing issue #322

adding a custom slot names arrow-icon to the main arrow. also adding same logic to the children arrows

[codecov[bot]]

Codecov Report

Merging #473 (9d5ac91) into master (9ae16b6) will decrease coverage by 0.36%. The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master     #473      +/-   ##
==========================================
- Coverage   97.68%   97.32%   -0.37%     
==========================================
  Files          27       27              
  Lines        1037     1046       +9     
  Branches      104      106       +2     
==========================================
+ Hits         1013     1018       +5     
- Misses          9       11       +2     
- Partials       15       17       +2     

Impacted Files	Coverage Δ
src/components/Control.vue	95.00% <50.00%> (-5.00%)	:arrow_down:
src/components/Option.vue	93.42% <66.66%> (-2.36%)	:arrow_down:

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[Woloja]

@riophae Hi! Can merge this feature?