Feature: Add command for installing an extension

[alistairjevans]

Need a simple command for adding an extension. Adding an extension involves modifying (or creating) an in-memory autostep.config.json with the new extension, then doing a basic extension load to verify the extension is loadable. If the extension load works, then write the new autostep config to disk.

Maybe add support for 'what-if?' mode to extension install in AutoStep.Extensions?

Example Usage:

Simple

> autostep add extension AutoStep.Web
Added AutoStep.Web; current version is 1.0.1.

Specific Version

> autostep add extension AutoStep.Web -v 1.1.0
Added AutoStep.Web; current version is 1.0.1

Allow Prerelease Extension

> autostep add extension AutoStep.Web --prerelease
Added AutoStep.Web; current version is 1.0.1-develop.83