support [gist:id=3254906] and fix bug

**CHANGES**

1. support secret gist . e.g.: https://gist.github.com/mckelvin/63c089cf1160117c535a
2. support syntax looks like `[gist:id=3254906]` for gist containing only one file.
3. fix a bug: any gist containing CJK characters will raise `'ascii' codec can't encode characters in position 0-3: ordinal not in range(128).` for Jinja2 is [using Unicode internally](http://jinja.pocoo.org/docs/api/#unicode ).