Allow sandbox plugins to call commands. Closes #186.

Ok, this one was a long time coming! This I think is going to turn into a really important feature, the ability for sandbox commands to call other commands, and chain them together. This is very Emacsy ;-)

So the first use of this is simply to invoke `Cursor:Left` after appending the newline, so that the cursor stays where it should, thus resolving that annoying bug #186