放宽引用gist的格式

讨论参见: http://www.v2ex.com/t/74249

```python
print gist("http://gist.github.com/123078.git")
<script src="http://gist.github.com/123078.js"></script>
print gist("http://gist.github.com/123078")
<script src="http://gist.github.com/123078.js"></script>
print gist("https://gist.github.com/123078")
<script src="https://gist.github.com/123078.js"></script>
print gist("https://gist.github.com/123078.git")
<script src="https://gist.github.com/123078.js"></script>
```