Make `gist-view-gist` customizable

Change `defvar` to `defcustom`.
binding to call browse-url on current gist

Extracts helper gist-current-url to share with gist-print-current-url.
Merge pull request #54 from ajtulloch/patch-1

Make `gist-view-gist` customizable
Merge pull request #56 from dgtized/gist-browse-current-url

binding to call browse-url on current gist