UPDATE delete to accept both id or full url

When we want do delete a gist, we can do
```
gost -D b9603076d61bb5287b52
```

But this will fail if full url is passed eg.
```
gost -D https://gist.github.com/b9603076d61bb5287b52
```

gost should be able to handle both.

REF: #26
Merge pull request #27 from MaximeD/delete-gist-with-full-url

UPDATE delete to accept both id or full url