Feat:[gatsby-cli] Allow plugins to create their own commands.

[nagiek]

Hello,

Given that gatsby-cli is built on yargs and seems quite extendible, it might be interesting to run an apiRunner to extend the cli with any commands offered by other plugins. Plugins could save their commands in a gatsby-cli.js file. These commands would then be loaded and included in createCli().

Example use cases might be:

• Selectively run part of the build process (i.e. sourceNodes from one specific source)
• Run a generator to create new components/pages from boilerplate
• Deploy to S3
• Build only some selected pages into a test directory to have a preview.

I am sure everyone is far too busy for this, but I wanted to make the suggestion.

Thank you