better file / tag management

I've noticed that when closing a tag, supertab triggers file completion:

![screen shot 2014-08-19 at 19 54 34](https://cloud.githubusercontent.com/assets/6401008/3970463/b3005b00-27ca-11e4-8a92-07075a52248f.png)

I've tried to improve the plugin so that it now closes the tag through omnifunc, while still triggering file completion when appropriate. I'm a vimscript newbie, so I'm not sure my changes are correct, but it seems they are working :)

this is my supertab configuration for reference:

```
let g:SuperTabDefaultCompletionType = 'context'

autocmd FileType *
            \ if &omnifunc != '' |
            \     call SuperTabChain(&omnifunc, "<c-p>") |
            \ endif
```