Add the possibility to set additional classes

Additional CSS classes names can be appended to the admonition name
using spaces as separators.

The following markdown:

    !!! note floatright
        This is a floating note.

Generates the following HTML code:

    <div class="admonition note floatright">
        <p class="admonition-title">Note</p>
        <p>This is a floating note.</p>
    </div>

1 files changed, 20 insertions, 2 deletions

diff --git a/docs/extensions/admonition.md b/docs/extensions/admonition.md
index 2d2634d..22a6aea 100644
--- a/docs/extensions/admonition.md
+++ b/docs/extensions/admonition.md
@@ -72,8 +72,26 @@ results in:
 </div>
 ```
 
-rST suggests the following `types`, but you're free to use whatever you want:
-    attention, caution, danger, error, hint, important, note, tip, warning.
+You can also provide additional CSS class names separated by spaces. The first
+class should be the "type." For example:
+
+```md
+!!! danger highlight blink "Don't try this at home"
+    ...
+```
+
+will render:
+
+```html
+<div class="admonition danger highlight blink">
+<p class="admonition-title">Don't try this at home</p>
+<p>...</p>
+</div>
+```
+
+rST suggests the following "types": `attention`, `caution`, `danger`, `error`,
+`hint`, `important`, `note`, `tip`, and `warning`; however, you're free to use
+whatever you want.
 
 Styling
 -------