Fix issue #21

I added the feature to optionally show the first indentation level. Can be switched on with:

```vim
let g:indentLine_showFirstIndentLevel=1
```

Default is `0`, meaning disabled. Here's an example:

![screenshot](https://f.cloud.github.com/assets/115889/258778/6ea59484-8ca8-11e2-9fd6-51d54c8ccc5a.png)

Please merge, if you like it!

Cheers, Patrick