Added lib_dir (Array<String>) support to configuration file.

[gpakosz]

Makes nanoc load code snippets from configurable directories.

[denisdefreyne]

Braces around arguments here:

@code_snippets.concat(code_snippets)
``

[denisdefreyne]

The create-site command (in lib/nanoc/cli/commands/create-site.rb) should be updated so that new configurations have the lib_dirs attribute.

[denisdefreyne]

👍 otherwise.

[gpakosz]

Updated. Not sure about create-site command since the current default configuration populated by the create-site command doesn't contain everything nanoc can do: e.g. static data sources

[denisdefreyne]

Yeah, maybe skipping the create-site changes is okay.

To do: update the documentation on the web site. (I will tackle that later when I write the 3.7 documentation.)