Replace absolute file path with base filename

Updating a gist with an absolute filepath yields the error "contents
can't be stored in subdirectories".  Instead of using the full path to
the file, we will instead use the base filename.
don't set buffer-file-name

after all, we're not visiting a file
Merge pull request #49 from keelerm84/bug/invalid-slot-type

gist-add-buffer: Invalid Slot Type
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