Add support for validating checksums

[malept]

Add a validate_checksum option, defaulted to false. When true, download a checksum from checksum_uri (formatted like binary_uri), parse the file and validate the tarball's integrity. For now, only handle sha256sum-style checksum files, as opposed to GPG-signed checksum files. The actual parsing and validating of the file would ideally exist in its own gem, as it's not specific to Thermite.