Add option for including stylesheets

[jwodder]

Add a command-line flag that, when set, causes all stylesheets specified with stylesheet or stylesheet_path to be collected and output in an html.stylesheets/latex.stylesheets field. For HTML, the contents of the field will be a series of either <style> blocks or <link> tags, depending on the setting of the embed_stylesheet option.

Possible names for the flag:

• --include-stylesheets
• --gather-stylesheets
• --collect-stylesheets
• --stylesheets-field