Adds gluegun plugin detection.

[skellock]

I have a suite of gluegun extensions incoming, and I'd like them to be available in ignite as well.

For example https://github.com/skellock/gluegun-prettier.

This makes these extensions auto-detectable.

yarn add --dev gluegun-prettier

context.prettier.reformat('/path/to/the/ugly.js')
// or
const pretty = context.prettier.format(` console\n.tab(   'lol'\t\t\t)\n\n\n`)
context.print.info(pretty)