Allow multiple pairs with same closing tag (helps with #108)

When given the following:

```lisp
(sp-local-pair 'ruby-mode "begin" "end")
(sp-local-pair 'ruby-mode "do" "end")
```

Only `begin` will work, because both `begin ... end` and `do ... end` have the same closing tag.

This solves this issue, by being more fuzzy when looking for sexp.