Add default configuration options

[tahirmt]

Adding a set of public properties to use as default for all tests. Allows configuration of these properties on a global level

• precision
• perceptualPrecision
• timeout
• snapshotSubdirectory to allow someone to change the default __Snapshots__ folder. Useful if you need to create multiple sets of snapshots depending on conditions.
• snapshotDirectory

Motivation

Providing an easy way to configure things like perceptualPrecision introduced in #424 when someone wants to apply it globally without having to write a ton of overrides. All of these options could be made configurable through the use of a plugin by writing a ton of extensions and passed in values. Snapshot location might be the toughest to do because we would need to reimplement the default behavior of a directory name at the same path as the file.

My work is similar to #641 but it allows for more customization