Prevent unnecessary copying of prototype property values to instances.

Allow `extname`, `layoutsDir`, and `partialsDir` property values to
bubble up to the the prototype unless an `ExpressHandlebars` instance is
explicitly configured overriding values.
Improve clarity of method implementations, expose more override "hooks".

Adds the following private methods: `_getPartialName()`,
`_renderTemplate()`, `_resolveLayoutPath()`. These provide additional
"hooks" to override the default behavior when in a pinch.
Bump version to 0.2.1.