Improve infrastructure for CLI options

[jgm]

Adding a CLI option is now a somewhat complex process, involving changes in a number of separate places.

• MANUAL.txt -- documentation for new option, both in the list of options and in the section on defaults files.
• Text.Pandoc.App.Opt -- new constructor for Opt and default value
• Text.Pandoc.App.CommandLineOptions -- the option parser
• Text.Pandoc.App or Text.Pandoc.App.OutputSettings -- handle the new option
• possibly in pandoc-server: Text.Pandoc.Server -- handle the new option

Some day it would be nice to create a slick system so everything can be in one place. I note that with --link-images, for example, not all of these things got done, and that might not be the only case.

[jgm]

For example, there is code in T.P.App.OutputSettings to set WriterOptions from Opts, and much of it is duplicated in pandoc-server.

[jgm]

Maybe pandoc-lua-engine could use generics to provide a compile-time check that it was marshalling all the fields of WriterOptions.