Add admonition styles for notes, tips, warnings, etc.

There are currently 4 styles: INFO, TIP, CAUTION, DANGER

In a doc they are used like:

NOTE: This is a note

1 files changed, 1 insertions, 0 deletions

diff --git a/Rules b/Rules
index ea8be954..ff7a5aef 100644
--- a/Rules
+++ b/Rules
@@ -63,6 +63,7 @@ compile '/**/*.md' do
       end
 
     filter :md_to_html_ext
+    filter :admonition
   else
     layout '/redirect.*'
   end