add "brackets" method for extract text between two chars

Add "brackets" method for extract text between two chars.

**brackets** _.brackets(string, [bracketCharOpen='(', bracketCharClose=')', include=false, recursive=false])

Returns array of strings inside `bracketCharOpen` and `bracketCharClose`.

```javascript
_.brackets('text outside brackets (and inside brackets), and outside (again)')
=> ["and inside brackets", "again"]

_.brackets('squire [brackets]', '[', ']', true)
=> ["[brackets]"]

_.brackets('find (nested brackets (recursively))', '[', ']', false, true)
=> ["nested brackets (recursively)", ["recursively"]]
I see a typo here. Accept my pull request dolgopolov/underscore.string#1 to fix it.