Add $s2bs-input-border-width variable

[AlexanderPoellmann]

I use a custom border-width on my buttons and form-controls of 3px using a custom variable which I called $input-border-width.

At the moment I override the select2-bootstrap-theme like so:

.select2-container--bootstrap {
    .select2-selection {
        border-width:$input-border-width;
    }

    .select2-search--dropdown {
        .select2-search__field {
            border-width:$input-border-width;
        }
    }

    .select2-dropdown {
        border-width:$input-border-width;
    }
}

It would be nice to have variable like $s2bs-input-border-width available in the first place, that spares me those additional lines.

That doesn't change anything for existing installs, yet enables users to customize the styles in a more consistent way.