simplify logic for resolving ext and engine

[jonschlinkert]

we need cleaner logic for auto-detecting the engine to use.

Hmm, maybe it's as simple as this:

• use the built-in default - engine-lodash - when no engine is defined by the user.
• if template.option('view engine', 'foo') is defined, use that as the default.
• use a .pickEngine() method to resolve the engine based on engine options, template params, etc.
• expose .pickEngine() on the options so it can be overridden by implementors/users.

template.option('pickEngine', function(env) {
  // env can be an object with template, options, locals, method details
  // do stuff and return an engine name
});

Exposing the option to use a custom .pickEngine() function allows us to implement simpler, more opinionated logic.

thoughts @doowb?