add helper functions at compile time

[vycb]

No description provided.

[vycb]

For example:
var index = ejs.compile(fs.readFileSync(__dirname + '/pub/index.html', 'utf8'),
{delimiter: '?', helpers: {echohtml: function(s){return s+" echohtml!"}}})

res.end(index({varhello: "Hello nodejs!"}));