Add `safeUrl`; disable `safeHtmlAttr`; rename `safeCSS` to `safeCss`

Add `safeUrl`; disable `safeHtmlAttr`; rename `safeCSS` to `safeCss`

- Add `safeUrl` template function (Fixes #347)
- Add TestSafeUrl() fashioned after @tatsushid great examples
- Disable `safeHtmlAttr` pending further discussions on its other
  use cases because `safeUrl` is a cleaner solution to #347.
  (There are also `safeJs` and `safeJsStr` that we could implement
  if there are legitimate demands for them.)
- Rename `safeCSS` to `safeCss` (to follow the convention of `safeHtml`)
- Add/expand documentation on `safeHtml`, `safeCss` and `safeUrl`