Added draft-mode plugin to remove page drafts, also added filters and tests to assemble menus based on page existence (Resolves #71)

[matthew-carroll]

Added draft-mode plugin to remove page drafts, also added filters and tests to assemble menus based on page existence (Resolves #71)

This PR includes:

• A new guide about draft mode
• Exclusion of pages during build when a property called draft is true (can be turned off with the plugin config)
• Jinja filters and tests that help construct menus in Jinja templates given that some pages might not exist
• shock build and shock serve now pass on any extra arguments so that an app might implement the concept of a "preview" mode when building or serving