Added CLI: `css-explain`

Just put together a CLI which allows one to score the CSS using css-explain.

### Installation

```bash
$ npm install -g css-explain
```

### Usage

```bash
$ css-explain my-styles.css # analyze one file
$ css-explain fork.css spoon.css # analyze as many files as you want
```

### Sample output

```
$ css-explain home.css screen.css 
Results for home.css:
    Results with score of 7:
        header#main-header div a, header#main-header div a:hover, #content #body 
        header, #content nav, #content #body .section-type .stuff p[class=bio], 
        #content #body .section-type .stuff span, #content nav a, #content #body 
        .section-type .stuff .photo img
    Results with score of 6:
        #content #body .section-type .stuff p[class=bio] > a:hover
    Results with score of 3:
        #content #body .section-type .stuff .photo .caption, #content #body 
        .section-type, #content #body .section-type a.person, #content #body 
        .section-type .stuff, #content #body .section-type a.person 
        span.name:hover, #content #body .section-type a.person span.name, #content 
        #body .section-type a.person:hover, #content #body .section-type .stuff 
        .photo
    Results with score of 2:
        header#main-header div#features-link, body:after, a:hover, a, 
        header#main-header div#leacocks-logo, header#main-header div#photos-link, 
        html
    Results with score of 1:
        span.namehovered, span.current-bio, header#main-header, #content #body 
        #landing, .current, .current:hover
Results for screen.css:
    Results with score of 7:
        ol li, header#main-header div, header#main-header div a, header#main-header 
        div a:hover, #content #body header, #content #body .section-type .stuff 
        .photo img, #content #body .section-type .stuff p[class=bio], .content p, 
        #content #body .section-type .stuff span, #content nav a, #content nav
    Results with score of 6:
        #content #body .section-type .stuff p[class=bio] > a, #content #body 
        .section-type .stuff p[class=bio] > a:hover
    Results with score of 3:
        #content #body .section-type, #content #body .section-type a.person, 
        #content #body .section-type .stuff, #content #body .section-type a.person 
        span.name:hover, #content #body .section-type a.person span.name, #content 
        #body .section-type a.person:hover, #content #body .section-type .stuff 
        .photo, #content #body .section-type .stuff .photo .caption::after
    Results with score of 2:
        h1, summary, section, nav, menu, hgroup, header, footer, figure, 
        figcaption, details, aside, article, h5, blockquote:after, 
        blockquote:before, q:after, q:before, blockquote, q, td, th, caption, 
        table, ul, ol, html, video, audio, mark, time, summary, section, ruby, 
        output, nav, menu, hgroup, header, footer, figcaption, figure, embed, 
        details, canvas, aside, article, td, th, tr, thead, tfoot, tbody, caption, 
        table, legend, label, a:hover, fieldset, li, a, ol, dd, dt, dl, center, i, 
        u, b, var, tt, sup, sub, strong, strike, small, samp, s, q, kbd, ins, img, 
        em, dfn, del, code, cite, big, address, acronym, abbr, a, pre, blockquote, 
        p, h6, h5, h4, h3, h2, h1, iframe, object, applet, span, div, body, html, 
        h6, h1, h2, h3, h4, h5, h6, html, header#main-header div#leacocks-logo, a, 
        a:hover, header, header#main-header div#photos-link, html, footer, ul, pre, 
        code, em, blockquote, ol, body, header#main-header div#campuspot-link, 
        header#main-header div#about-us-link, header#main-header div#features-link, 
        body:after
    Results with score of 1:
        .current, .current:hover, .line-break, .site-description, .date, #content 
        #body, span.namehovered, span.current-bio, #content #body #landing
```

### Future

In the future, it'd be cool to have:

* ability for file args to be directories and get all `.css` files in that dir
* colorized output:
    * red = bad score
    * yellow = ok score
    * green = good score

Let me know what you think, @josh!