Add porter install command

[carolynvs]

This is the skeleton for the install command.

$ porter bundle install --help
Install a bundle

Usage:
  porter install [flags]

Examples:
  porter install
  porter install --insecure
  porter install --file myapp/bundle.json
  porter install --name MyAppInDev
  porter install --param-file base-values.txt --param-file dev-values.txt --param test-mode=true --param header-color=blue
  porter install --cred azure --cred kubernetes


Flags:
  -c, --cred strings         Credential to use when installing the bundle. May be either a named set of credentials or a filepath.
  -f, --file string          Path to the CNAB definition to install (default "bundle.json")
  -h, --help                 help for install
      --insecure             Allow installing untrusted bundles
      --name string          Name of the claim, defaults to the name of the bundle
      --param strings        Define an individual parameter in the form NAME=VALUE. Overrides parameters set with the same name using --param-file. May be specified multiple times.
      --param-file strings   Path to a parameters definition file for the bundle, each line in the form of NAME=VALUE. May be specified multiple times.

• alias: porter install
• --insecure is required until porter build should always create secure bundles #224 is implemented

Part of #106.

[vdice]

Only comment around flag usage. Else, LGTM!