this is the first step in building a new `zola index` command that allows building search indexes as a stand-alone task.

the new Subcommand in src/cli.rs outlines the planned api.

the biggest thing it will be providing compared to current code is optional indexing with tantivy.

this commit doesn't add tantivy or any new functionality other than allowing the existing search indexing to be performed as a subcommand. (passing --index-type tantivy will trigger unimplemented!() panic)

* Add check subcommand

* Add some brief documentation for the check subcommand

* Start working on parallel link checks

* Check all external links in Site

* Return *all* dead links in site