add part function in html.go
add part function in html.go

layout.html 
 <head>
    {{ part "css" }}
</head>

home/index.html
{{ define "home/index-css"}}
<link rel="stylesheet" type="text/css" href="/public/css/home/index.css" />
{{ end }} 

in html/temple  the defined block is global variable， so the way of "{{ block "css" }}" in layout and "{{ define "css"}}{{end}}" in home/index.html and home/about.html   just one definition work, others will be over coved。  No use！！！

This part function like extend the block in layout.

Former-commit-id: c3e26a8227808cf2d1dc7121bcffe1a98ce3e678
kataras/iris,kataras/iris,kataras/iris,kataras/iris,kataras/gapi,kataras/gapi