Add CSS.next var and calc style declarations. Add style property to package.json.

This will allow people using rework (or one of its many derivatives) to pull in typebase into their stack. It also serves as a more 'future-looking' implementation as rework-calc and rework-vars both track the CSS specifications as best as possible (but don't polyfill the impossible stuff).

I also added a 'style' property to the package.json file so people could use typebase.css with npm if they'd like. (prior art: https://github.com/necolas/normalize.css/blob/master/package.json#L5 )

Here is the contents of the compiled file if you'd like to avoid learning how to use rework:

```css
/*! Typebase.rework.css v0.1.0 | MIT License */

/* Typesetting variables. Edit these!
// This requires rework-vars and rework-calc */

:root {
  /* Base font size in pixels. This would result in 22px on desktop */
  /* How large the line height is as a multiple of font size */
  /* Rate of growth for headings
     I actually like this to be slightly smaller then the leading. Makes for tight headings.
  */
}

/* Setup */

html {
  /* Change default typefaces here */
  font-family: serif;
  font-size: 22px;
  font-size: 137.5%;
  /* Make everything look a little nicer in webkit */
  -webkit-font-smoothing: antialiased;
  /* -webkit-text-size-adjust: auto
    // -moz-text-size-adjust: auto
    // -ms-text-size-adjust: auto
    // -o-text-size-adjust: auto
    // text-size-adjust: auto */
}

/* Copy & Lists */

p {
  line-height: 1.5rem;
  margin-top: 1.5rem;
  margin-bottom: 0;
}

ul,
ol {
  margin-top: 1.5rem;
  margin-bottom: 1.5rem;
}

ul li,
ol li {
  line-height: 1.5rem;
}

ul ul,
ol ul,
ul ol,
ol ol {
  margin-top: 0;
  margin-bottom: 0;
}

blockquote {
  line-height: 1.5rem;
  margin-top: 1.5rem;
  margin-bottom: 1.5rem;
}

/* Headings */

h1,
h2,
h3,
h4,
h5,
h6 {
  /* Change heading typefaces here */
  font-family: sans-serif;
  margin-top: 1.5rem;
  margin-bottom: 0;
  line-height: 1.5rem;
}

h1 {
  font-size: 4.242rem;
  line-height: 4.5rem;
  margin-top: 3rem;
}

h2 {
  font-size: 2.828rem;
  line-height: 3rem;
  margin-top: 3rem;
}

h3 {
  font-size: 1.414rem;
}

h4 {
  font-size: 0.707rem;
}

h5 {
  font-size: 0.4713333333333333rem;
}

h6 {
  font-size: 0.3535rem;
}

/* Tables */

table {
  margin-top: 1.5rem;
  border-spacing: 0px;
  border-collapse: collapse;
}

td,
th {
  padding: 0;
  line-height: 33px;
}

/* Code blocks */

code {
  /* Forces text to constrain to the line-height. Not ideal, but works. */
  vertical-align: bottom;
}

/* Leading paragraph text */

.lead {
  font-size: 1.414rem;
}

/* Hug a the block above you */

.hug {
  margin-top: 0;
}
```