relax regex for blocks

this allows you to use e.g gulp-rev`s rev-manifest.json file to replace
assets paths.

e,g, for a `rev-manifest.json` which can be passed to `htmlreplace(json)`

````json
{
  "javascripts/apps.js": "javascripts/apps-37a1f394.js",
  "stylesheets/index.css": "stylesheets/index-b7b7ad08.css",
}
````