[Extensions] Make parser options changeable

Add flag enum for the possible parser options. Use the passed flags
instead of hardcoded value when calling mkd_string() and mkd_compile().
[Extensions] Add parser options to document

Add parser options to Discount::Document constructor. Pass the options to
the Parser::parseString() call.
[Extensions] Add autolinkEnabled to Options class

Add property "autolinkEnabled" to Options class.
[Extensions] Autolink option

- Add sub-menu "Markdown Extensions" with entry "Autolink" to menu "Extras"
- Pass Options to HtmlPreviewGenerator
- Set parser option Autolink based on state of new menu entry
[Extensions] Strikethrough option

Add menu entry to "Markdown Extensions" which controls the parser option
Parser::NoStrikethroughOption.
[Extensions] Refactor HtmlPreviewGenerator

Move assembling the parser options into its own method.
[Extensions] Alphabetic lists option

Add menu entry to "Markdown Extensions" that controls the parser option
Parser::NoAlphabeticListOption.
[Extensions] Definition lists option

Add menu entry to "Markdown Extensions" that controls the parser option
Parser::NoDefinitionListOption.
[Extensions] SmartyPants option

Add menu entry to "Markdown Extensions" that controls the parser option
Parser::NoSmartypantsOption.
Merge branch 'feature/markdown-extensions-#30' into develop