Add support for package metadata

[ldodds]

Add support for parsing package.json from the directory of queries.

The package metadata should include the following:

• title of the package
• summary of the package
• list of additional files to process and include as markdown
• list of additional files to copy to output, e.g. images
• default list of @endpoint values, which will be used if none are set on queries
• default list of @author values, which will be used if none are set on queries
• default list of @tag values, which will be used if none are set on queries

[ldodds]

The majority of this is now implement. All that is remaining is:

• list of additional files to copy to output, e.g. images

Might also want to improve extra-files parameter to support page title.

[ldodds]

The only item remaining on this list is copying additional files to the output.