doT: support partials

[HoKangInfo]

koa-views use doT

app.use(views(__dirname + '/views', {
options : {
// _def: {user: 'my name</span'
partials: {user: './partials/user'}
},
map: { html: 'dot' }
}))
support load doT def from file.

v.0.14.5
-1032 var tmpl = cache(options) || cache(options, engine.compile(str, options && options._def));
+1032 var tmpl = cache(options) || cache(options, engine.compile(str, options && Object.assign({}, options._def, options.partials)));