Support GitHub Enterprise

if you use jist with GitHub Enterprise, you shuold run with `--login` at first.
Because GHE needs OAuth token.

Usage
-----------

In terminal:

```sh
$ export GHE_URL=<your_ghe_url> 
$ jist --login
$ jist a.rb
```

If you want to stop using GHE, run `$ unset GHE_URL`.

TODO:
- [x] tests
- [ ] var name review