Add a subcommand to dotnetsuggest for retrieving a suggestion command from Nuget or equivalent

Add an `install` command to `dotnetsuggest` that:
Leveraging dotnet global tools downloads a tool that has a command for suggestions. Once install, register the global tools command so that it is associated with a command to which we are adding suggestions.

Proposed command line (initial):
`dotnetsuggest install -tool -commandPath -suggestionCommand `
`toolPackageName` The nuget package name that provides command suggestions
`commandPath` The path to the command for which we are registering completions.
`suggestionCommand` The command to execute in order to return suggestions.

**Future:**
* Specify the suggestionCommand in the tool package nuspec file so that it isn't required to specify it on the command line.
* Support a static registration file rather than just a global tool for provide suggestion completion
* Update global tools to register suggestion completion automatically when the global tool is installed either using the global tool itself or a dependent Nuget package that provides suggestion completion.

**Notes**
* By supporting a global tool for suggestion completion immediately we can use System.CommandLine to build the tool immediately - no need to worry about serialization.