[Suggestion] add a variables file, allow modifying the variables used in the scss files

[WilliamStam]

im not a fan of the forcing of using secondary for many of the label colours etc. wouldnt it be better if you included a _variables.scss file similar to how bootstrap does it with !default? that way we can override the defaults with say $select2-theme-bootstrap4-secondary: $gray;

$select2-theme-bootstrap4-input-bg: $input-bg !default;
$select2-theme-bootstrap4-input-border-radius: $input-border-radius !default;
$select2-theme-bootstrap4-input-box-shadow: $input-box-shadow !default;
$select2-theme-bootstrap4-input-transition: $input-transition !default;
$select2-theme-bootstrap4-input-focus-border-color: $input-focus-border-color !default;
$select2-theme-bootstrap4-input-focus-box-shadow: $input-focus-box-shadow !default;
$select2-theme-bootstrap4-input-disabled-bg: $input-disabled-bg !default;

$select2-theme-bootstrap4-secondary: $secondary !default;

should be as easy as finding $ and then just prefixing them with a prefix and setting up a _variables.scss file

[rodrigo-arias]

Totally in favor of this approach

[tagliala]

I would like to do this in #67