Validate command

[squaremo]

$ jk help validate
Validate configuration files

Usage:
  jk validate <file>... [flags]

Examples:

  validating YAML files using a module's default export
    jk validate -m '@example.com/validate' *.{yaml,yml}

  validating YAMLs and JSONs using a local script's default export
    jk validate ./valid.js *.{yaml,yml,json}

  validating a specific file with an inline validation function
    jk validate -c 'v => v.name === "correctName"' config.json


Flags:
  -d, --emit-dependencies         emit script dependencies
  -c, --exec                      treat first argument as specifying literal JavaScript to execute
  -h, --help                      help for validate
  -m, --module                    treat first argument as specifying a module to load
  -o, --output-directory string   where to output generated files
  -p, --parameter name=value      set input parameters
  -f, --parameters filename       load parameters from a JSON or YAML file
  -v, --verbose                   verbose output

Fixes: #235

[squaremo]

Stretch goals (for the record; not intending them to be done in the PR):

• global validation -- validation that acts on the whole set of objects (how to provide an argument for it?)
• -r treat the file arguments as directories into which to descend

-r makes most sense if there's global validation -- otherwise, you could just get the list of files with find | xargs.

[squaremo]

(rebased and moved support JS to cmd/ as with #284)

[squaremo]

@dlespiau How do you feel about this new subcommand?

[dlespiau]

Looks useful to me! I like that we didn't go with a recursive mode, leaving the choice of which files to select to the command line (which could be a find + xargs).