What is this?

While working on #305, the decision was made to have the config schema for snapshot options included in @percy/core since that is where the options are consumed. Initially however, core only registered the schema while the snapshot command used it to validate snapshot listing options.

With #406 refactoring the merge and validate utils, the getSnapshotConfig helper can now be refactored to utilize their new features to better merge and now validate options! This brings config validation to all of the SDKs as warnings logged whenever percySnapshot is called with unrecognized or invalid options.

Speaking of invalid options, this PR also adds more restrictions to existing config options:

• The widths and minHeight options have min and max values. These values are derived from the upload command clamping, which itself was derived from our API restrictions.
• Hostnames in allowedHostnames must be actual hostnames, meaning no protocols and no paths. Including protocols and paths never worked and this validation will help clear up that confusion.
• The networkIdleTimeout should never really be larger than 500ms, but is restricted to 750ms to give it some breathing room. This restriction is imposed because long timeouts cause slow tests and don't actually address the underlying network issues.
• Discovery concurrency has a minimum value of 1 because having a negative or zero value causes the queue to lock up.

For new per-snapshot validations other restrictions are also imposed:

• Disallowed capture options, like waitForSelector, when performing discovery for an existing domSnapshot.
• The waitForTimeout option must be between 1ms and 30000ms.
• The execute option must be a string or function.
• For additionalSnapshots, a name, prefix, or suffix is required.
• For additionalSnapshots, a prefix or suffix cannot be provided with a name.

Tests were updated accordingly and a couple things that were previously taken care of in the config helper are now taken care of where necessary, such as defaulting an additional snapshot name using a prefix/suffix.

This will close #389