highlight search keyword in current page

highlight.js based on http://bartaz.github.io/sandbox.js/jquery.highlight.html#how.
add search-highlight.less to custom style for different theme,default is
yellow,same as the osx's search result style.
when toggle the search input element to close,current page's highlight
elements will be cleared.
use space to split search keywords,highlight all the results.
use regex to match the content,ignorance case.
havn't add any options to custom the default behaver,see if any issue
announce this.