Add referenceBuildInfoDir and exclude options

[ericglau]

Fixes #56

Adds options to make use of OpenZeppelin/openzeppelin-upgrades#1062 and OpenZeppelin/openzeppelin-upgrades#1065

• Adds referenceBuildInfoDir option which takes the path to an older build info directory, which can contain the previous version of a contract to use as a reference for storage layout comparisons
  • When this is provided, that older build info directory should be referenced in the @custom:oz-upgrades-from annotation or using the referenceContract option, for example by using: build-info-v1/MyContract
• Adds exclude option which takes a string array of glob patterns, from which to exclude contracts from validations
• Updated and reorganized documentation