Normalize paths in deprecation messages

[sambostock]

Summary

This (tentatively) adds support for registering deprecation message normalizers, and provides built in normalization of paths by looking for the following strings:

• appearances of Dir.pwd, which are replaced with #{Dir.pwd}
• appearances of any of Gem.path's paths, which are replaced with #{Gem.path[index]}, where index is the index of the path in the Gem.path array

Replacements are made to look like interpolation, so it is clear what they represent.

This should resolve #48.

⚠️ Before Merging

• Validate approach
  • Should normalizers be expected to mutate the message (to avoid duplicate string allocations), or is it clearer if they return the normalized message (at the cost of creating duplicate objects)?
• Write tests
• Consider other safe built in replacements that may have been missed.
  • What happens if a gem is sourced from somewhere non standard? e.g. GitHub? Path?
• Document new config in README.md
• Update CHANGELOG.md