* Pre-parse platform string with StringSliceVar
This allows users to declare --platform multiple times and have the
values appended, i.e.:
ko build --platform=linux/amd64 --platform=linux/arm64
is equivalent to
ko build --platform=linux/amd64,linux/arm64
As a side effect, platformMatcher.spec and gobuildOpener.platforms are
now of type []string (instead of string) to maintain structure of
information from flag parsing.
* Adjust comments and styling for clarity.
* The flag --platform is now of type strings.
Internally cobra/pflag defines StringSliceVar as "strings" whereas
StringVar is defined as "string".
This change is updated by running hack/update-codegen.sh script.
* Add backwards compatibility for WithPlatforms function signature
Update comments to reflect implementation as well.
* Fix syntax failure on unit test
