allow string values in cons for :bind keywords

allow string values in cons for :bind keywords

It is possible with `bind-key` and `define-key` (and also `bind-chord`
and `key-chord-define`) to define a binding to a string's value, i.e:

``` elisp
(bind-key "C-;" "the ")
(bind-chord "^^" "λ")
```

This adds an option for `(use-package-normalize-pairs)` that allows
string values to be given with the `:bind` (and also `:chord`) keywords
to expand into these definitions.