partially centralize issues url

remove literal "https://github.com/npm/npm/issues" where possible
  create new config 'bugs' defaulted to "https://github.com/npm/npm/issues"
  use '@ISSUES' in doc/**/*.md in place of literal
  change doc-build.sh to replace @ISSUES@ with `npm config get bugs`
  ensure that "." is escaped in generated man pages
  use npm.config.get("bugs") instead of literal in error-handler.js

There are still 3 independent places where "https://github.com/npm/npm/issues" appears
as a literal:

  package.json - as 'url' for 'bugs'
  README.md - near the end, "When you find issues, please report them"
  lib/config/defaults.js - as default value for 'bugs'

So it's not quite DRY, but a bit less damp.