tools: enable more remark-lint rules

New rules:
 1. rule-style
 2. strong-marker
 3. no-shell-dollars
 4. no-inline-padding
 5. code-block-style
 6. no-multiple-toplevel-headings

Fixes to the existing files applied.

PR-URL: https://github.com/nodejs/node/pull/8708


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: James M Snell <jasnell@gmail.com>