Allow `shell_command`'s `command` field to be linted/formatted

[chrisjrn]

When BUILD files are linted/fixed, run the contents of the command field through a linter/fixer to ensure that quality standards are met.

[kaos]

fyi The way we've done this for the instructions field on the docker_image target is to have the linter consume the Dockerfile instructions from the field as generated sources.