escape-string-regexp known bugs

With the current behavior of escaping `-`, there is a problem if used in a Unicode regex. You can see that: ```js const esc = require('escape-string-regexp'); new RegExp(esc('-'), 'u'); ``` gives: > Invalid regular expression: /\-/: Invalid escape because this does also: ```js new RegExp('\\-', 'u'); ``` Would seem to call for a Unicode mode? (The hyphen would still need escaping (and be safe to use) within a character class though.) I don't know whether you'd want to get into such parsing, but just sharing as it is something I came across.