Support for Packer plugin API version 5 (Packer 1.5.6)

This release adds support for the current version of the Packer plugin API (version 5),
and Packer 1.5.6.

Basic support for resolving packer user variables.

This release contains the following:

New features:

• Basic support for resolving packer user variables. If resolution fails,
  then the plugin will attempt to locate the file in the project directory

Bug fixes:

• Actually enforce maximum file save size for local files

Initial release.

v0.0.1

On second thought, keep 'osCategory' private.