Support -f/--file flag in argocd app add

[merenbach]

Summary

This adds an -f flag to (mostly) mirror Kubernetes' kubectl create -f flag, and in the process resolves #31.

Problem

Currently a GitHub repo and a path to a manifest inside the repo are required. It would be nice to be able to use a -f flag to point to a local file or URL containing a Kubernetes Application manifest that can be used directly. The repo and path can be recovered from inside the file.

Solution

• Add util methods for reading remote/local files.
• Add tests for remote/local file reading methods.
• Add util method for unmarshaling byte data into an argoappv1.Application struct.
• Update NewApplicationAddCommand:
  1. Don't require path and repo args at the FlagSet level.
  2. Accept an -f/--file flag that contains either a local path or a remote URL.
  3. If the flag points to a valid JSON or YAML file, try unmarshaling it with the aforementioned util method.
  4. Error out if this fails.

Notes

@alexmt and any other reviewers:

• Because the K8s manifest is assumed to contain a name in the metedata, this PR takes the liberty of removing the APPNAME positional parameter and replaces it with a named parameter.
• File extensions are not checked, since it's really just the contents we're interested in. This may or may not match kubectl create -f directly.
• kubectl create -f also allows local directories to be specified. Since Application manifests are small for now and not generally divided between files, directories are not currently supported.

[alexmt]

LGTM. I've added two minor comments. Please fix and merge

[alexmt]

log.Fatal already executes os.Exit(1). Can you please remove this line?

[alexmt]

Nitpicking. Please run goimports -w cmd/argocd/commands/util.go. We usually automatically run goimports on file save, so next person might get unexpected changes.

[merenbach]

@alexmt thank you! The requested changes have been made, and per https://godoc.org/golang.org/x/tools/cmd/goimports I've updated my .emacs.