What is this?

This PR refactors and tweaks the percy snapshot command to be a little more friendly.

First, a page listing no longer needs to be a list of objects containing at least a url property. Now the file can contain just a list of URLs in the appropriate format:

# snapshots.yml
- https://example.com
- https://example.com/about
- https://example.com/contact

Second, the --base-url flag was made to work with page listings in addition to the existing static directory usage. Since usage differs between page listings and static directories however, some other small adjustments were necessary.

For static directories, the base URL should be the base PATHNAME that the site would be hosted at. Most sites are hosted at the domain root (/), however some sites might be hosted at other paths (e.g. /blog). This requirement has been present since the @percy/agent snapshot command and continues to remain a requirement.

For page listings, the base URL should be the entire base URL including the protocol and hostname. For example, the above list of URLs can be simplified to remove the common base URL:

# snapshots.yml
- /
- /about
- /contact

$ percy snapshot --base-url https://example.com snapshots.yml

To prevent mixing up the static.base-url configuration option with the page listing flag, a couple other changes were also made. The flag is no longer auto-mapped to the configuration option via the percyrc flag property. Instead, it is passed along to the .percyrc() method which now accepts an initial overrides argument.

The static.base-url configuration option was no longer being checked for the leading forward slash (only the flag is checked by the command now). A pattern requirement was added to the schema, and custom errors messages were added to config validation to print a better error message than what is normally printed for pattern failures. We can also utilize this pattern to do other better config validations in the future such as for certain discovery options.

The --dry-run flag will now print a better list of snapshots, however the default name in core is only created when calling the snapshot method. The default name was also added here so dry runs can properly print it. When adding the default name here, I discovered a small bug in the core implementation in which uri.path needed to be replaced with uri.pathname.