Table of contents and cover functionality

There is a static build of wkhtmltopdf that allows one to add table of contents and cover options. Since I wanted that and could not find anything in the your code that supports its particular way of handling it these options, I wrote it myself. If it is supported and I missed something, then I guess you can ignore this request.

I included also a small change to line 54 of lib/pdfkit/pdfkit.rb because it was screwing up syntax highlighting in Emacs (at least with my config.) All the tests pass with this change.

I add to config options to PDFKit::Configuration:

```
  config.toc_options = {
    :xsl_style_sheet => "toc.xsl"
  }

  config.cover_options = {
    :file => "cover.html"
  }
```

This can also be added when a PDFKit object is initialized:

```
PDFKit.new('...', :toc_options => {:xsl_style_sheet => 'toc.xsl'}, :cover_options => {:file => 'cover.html'})
```

(The :file flag is taken out of the command line output because the output needs to be ```cover cover.html```.)


Thanks,
Brian