Multitask compliant and some new options

Hi,

great job with this plugin, it's an interesting project, not giving too fixed commenting rules. I've made some changes:

* make it compliant to grunt's multitask tasks and make it handle `files` as an array
* added an option to **include/exclude readme.md** or even specify a path to it
* added **start/end comment's marker** options (as regular expressions) so that you can specify the comment's format you want to use (for example i use [this](https://github.com/necolas/idiomatic-css#comments)
* added an option to **exclude files where no comments are found**
* added an option to **specify a custom base URL fo navigation**, useful when you're creating a styleguide with path `public/styleguide/` but the styleguide will be served from `/styleguide/`.
* I've also added a section in the template to **show the CSS/SCSS source** along with result and HTML source.

Except from the task format (which now requires a target to be specified), i've tried to maintain compatibility with current version (your example still works perfectly)

I've updated the documentation including what's changed.