Make zenburn the default colorscheme

This commit also adds better support for constant.number in the
syntax files.

runtime/syntax/sh.micro

@@ -2,7 +2,7 @@ syntax "shell" "\.sh$" "\.bash" "\.bashrc" "bashrc" "\.bash_aliases" "bash_alias
 header "^#!.*/(env +)?(ba)?sh( |$)"
 
 # Numbers
-color constant "\b[0-9]+\b"
+color constant.number "\b[0-9]+\b"
 
 # Conditionals and control flow
 color statement "\b(case|do|done|elif|else|esac|exit|fi|for|function|if|in|local|read|return|select|shift|then|time|until|while)\b"