Overview

This PR introduces the infrastructure for REPL commands. Fixes #224 and #237. The design is as discussed in #583.

The command is defined as follows:

public interface IReplCommand
 {
     string CommandName { get; }
     object Execute(IScriptExecutor repl, object[] args);
 }

And can either execute arbitrary code, or interact with the REPL (has access to IScriptExecutor). If it returns the value, it's printed to the REPL too. It should now enable easy extensiblity i.e. do something like requested in #402 or #427

The interesting thing is that you can also pass context variables to the REPL. For example:

   > var x = 1;
   > :myCommand hello x

Will pass hello string and 1 to command. This is done by evaluating the argument with the ScriptEngine- if it returns a value, use it, otherwise assume it's just a string.

Commands included

Converted the existing REPL commands to this approach:

• CwdCommand (show working directory)
• CdCommand (change workign directory)
• ClearCommand (cls)
• ResetCommand (reset executor)

Also added InstallCommand - this fixes #609.
You can now do:

• :install myPackage
• :install myPackage 0.3
• :install myPackagae 0.3 -pre

Extra points

• currently the infra loads all commands it can find in the AppDomain. The idea was that as long as you have a DLL with IReplCommand, it should be discovered. This is a bit too invasive, so we may want to change it for a better approach. At the moment it's not integrated with Modules, but it should be trivial to do.
• right now all of the commands are loaded with : - I think it's actually better to have a uniform way rather than have a couple of commands starting with # because Roslyn does that, and have the rest start with :. But of course it would be possible to have "system" commands that start with #.
• The code to parse command is now embedded into the Repl class. There are better ways to do it but I didn't want to do too much in one PR. See issue Make REPL injectable and overrideable like other services #434 - I think this should be tackled next.
• I have plenty of other examples if there is some interest in extending it further - some mentioned in Using scriptcs as a liveable shell? #583 already - i.e. HttpCommand or Command to build other commands inline :-)

Let me know what you think!