Allow specifying the root directory in the CLI arguments

[sandhose]

It would be great to be able to set the root directory of the site when building it, e.g zola build --root my_website/.
A use case of this would be in scripts & some build systems. I tried to make Bazel to run Zola, but I had to write a wrapper that cd into the site directory before building it.

To do this, Zola should:

• introduce a new (global?) argument, defaulting to the current working directory (in src/cli.rs)
• use it where current_dir is used
  • in the check command
  • in the build command
  • in the serve command

[Keats]

Anyone wants to pick that up?