Add "view options" settings for rendering engines

[nvlled]

Avoids cluttering app.locals, and is consistent with the assignment of the "view engine" settings:

app.set("view options", {
    basedir: "somedir"
});
app.set("view engine", "pug");

The intent is also stated more clearly in comparison to:

app.locals.basedir = "somedir"

[dougwilson]

Seems reasonable. Would this only apply to the set default view engine, or to all view engines (and if so, how would conflicting options be resolved)? Please add some tests when you find the time 👍

[nvlled]

Yeah, it applies to all view engines, although I've only looked and modified the app.render code, so I may not be entirely sure. ~~It's basically an alias for the app.locals.~~ I'll do a double check.

Edit: Yeah, it applies to all view engines, not just the default one. It merges the options in the following order:

1. app.locals
2. app.settings["view options"] (this is what is added)
3. renderOptions (from app.render(name, renderOptions, callback) { ... })

The latter ones override the former. I'm not sure if the added test suffice or they are in the right place.