Make a series of sweeping changes to make :ALEInfo more useful.
1. Deprecate :ALEInfoToClipboard and support :ALEInfo -clipboard
2. Permit :ALEInfo -clip as a shorthand for :ALEInfo -clipboard
3. Support :ALEInfo -preview to render in the preview window
4. Support :ALEInfo -echo for the classic :ALEInfo mode
5. Change the default mode to 'preview', and make it configurable
6. Add syntax highlighting for ALEInfo in preview mode
7. Add a convenience to look up documentatation that explains itself
8. Don't show an empty 'Linter Variables' section
Default navigation for commands that jump to new locations has been
implemented with the `ale_default_navigation` variable, and all commands
that jump to locations now support `-tab`, `-split`, or `-vsplit`
arguments for overriding the default navigation behavior.
