Add line-height as optional 3rd argument

Author: ahocevar

README.md

@@ -10,8 +10,8 @@ The ["Klokantech Noto Sans"](https://github.com/klokantech/klokantech-gl-fonts)
 
 ```js
 var parseFont = require('mapbox-to-css-font');
-parseFont('Open Sans Regular', 16);
-// returns 'normal 400 16px "Open Sans"'
+parseFont('Open Sans Regular', 16, 1.2);
+// returns 'normal 400 16px/1.2 "Open Sans"'
 ```
 
 ## API
@@ -22,4 +22,6 @@ parseFont('Open Sans Regular', 16);
 
 -  `size` **[number](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number)** Font size in pixels.
 
-Returns **[string](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String)** CSS font definition, e.g. `'normal 400 16px "Open Sans"'`.
+- `lineHeight` **[string](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String)|[number](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number)** Line height as css [line-height](https://developer.mozilla.org/en-US/docs/Web/CSS/line-height). Optional.
+
+Returns **[string](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String)** CSS font definition, e.g. `'normal 400 16px/1.2 "Open Sans"'`.

index.js

@@ -29,7 +29,7 @@ var italicRE = /(italic|oblique)$/i;
 
 var fontCache = {};
 
-module.exports = function(fonts, size) {
+module.exports = function(fonts, size, lineHeight) {
   var cssData = fontCache[fonts];
   if (!cssData) {
     if (!Array.isArray(fonts)) {
@@ -68,8 +68,8 @@ module.exports = function(fonts, size) {
       }
       fontFamilies.push(fontFamily);
     }
-    // CSS font property: font-style font-weight font-size font-family
+    // CSS font property: font-style font-weight font-size/line-height font-family
     cssData = fontCache[fonts] = [style, weight, fontFamilies];
   }
-  return cssData[0] + sp + cssData[1] + sp + size + 'px' + sp + cssData[2];
+  return cssData[0] + sp + cssData[1] + sp + size + 'px' + (lineHeight ? '/' + lineHeight : '') + sp + cssData[2];
 }