Update# add auto fill filename feature to support ejs include

Hello, I add some code to let ejs can using "include" feature.
I let it auto fill `filename` options if not specify, and ejs can find partials correctly when use `<% include _partials %>`

I also test below setting can skip "_" prefix ejs file but still provide ejs include feature.
```js
gulp.src(["app/**/*.ejs", "!app/**/_*.ejs"])
       .pipe(ejs())
       .pipe(gulp.dest("public"))
```