Add documentation for font size unit options

Commit introduces documentation for options relating to font size unit:
 - 'fontsizeunit' in toolbar
 - 'fontSizeUnit' as a summernote method call

Documentation for fontSize has also been changed accordingly.

Author: jckmgns

01-deep-dive.md

@@ -58,6 +58,7 @@ You can compose a toolbar with pre-shipped buttons.
 * Font Style
   * `fontname`: set font family
   * `fontsize`: set font size
+  * `fontsizeunit`: set font size unit
   * `color`: set foreground and background color
   * `forecolor`: set foreground color
   * `backcolor`: set background color
@@ -169,6 +170,15 @@ $('#summernote').summernote({
 });
 {% endhighlight %}
 
+### Custom font size units
+You can set the available font size units with the `fontSizeUnits` option.
+
+{% highlight javascript %}
+$('#summernote').summernote({
+  fontSizeUnits: ['px', 'pt']
+});
+{% endhighlight %}
+
 ### Custom line heights
 You can override the default list and specify a custom one.
 
@@ -413,10 +423,18 @@ $('#summernote').summernote('fontName', 'Arial');
 Set font size.
 
 {% highlight javascript %}
-// @param {Number} font size - unit is px
+// @param {Number} font size - unit is determined by selected font size unit.
 $('#summernote').summernote('fontSize', 20);
 {% endhighlight %}
 
+### fontSizeUnit
+Set font size unit.
+
+{% highlight javascript %}
+// @param {String} font size unit - defaults to px.
+$('#summernote').summernote('fontSizeUnit', 'pt');
+{% endhighlight %}
+
 ### removeFormat
 Clean a style.