Adds selector utils we're currently using on Carnival

{TICKET}: (brief description / PR title)

Status: **Opened for visibility**
Reviewers: **@Bielousov @scalvert**

## Adds the following selector utils
- getSelectedOption: returns the selected option inside a select
- outerHeight: calculates the height of an element including margins and borders
- removeEmpty: removes empty elements in the same selection
- swap: swaps the selected element with a specified tag
- prevUntil: traverses siblings backwards and returns all that match a selector until a different element is met
- nextUntil: traverses siblings forwards and returns all that match a selector until a different element is met

## Tasks to complete before merging
- [ ] write tests
- [ ] update README

This returns an option element that has the same value as the dropdown's .val().
So you would be able to get a text or any data attribute or anything else for the currently selected option. 

This function does what $('select option[selected]') should do, however that selector doesn't work as expected if dropdown value has been changed in the following manner: $('select').val(2);