Support single-url scraping

Add a new command to support scraping a _single_ url and all its comments. E.g.,

```
./bin/scrape-url <URL> <Workspace> <ThemeName> <PageName>
```

This will:
-  create directory `Workspace`/`ThemeName` if it doesn't already exist.
-  Save `URL` to `PageName`
-  Save all the assets of `URL` to the `ThemeName` directory, and
-  Rewrite html to point to them
