Add plugin support

[nfischer]

It would be nice if there was support for incorporating ShellJS plugins into shx, like so:

$ shx open file.txt
$ shx git status # assuming there is a git plugin
$ shx mktemp foo.XXXXXX

Since shx blacklists certain commands, it might be good to have some option when creating the ShellJS plugin about whether it will be exposed to shx. This would also get rid of the need for a literal blacklist, since it would then be redundant.

[ariporad]

Agreed. We could just require everything we can with a module like load-plugins.

[nfischer]

Will that module work with ShellJS plugins? It looks like it's designed for a few specific task runners.

[ariporad]

I think so, AFAICT it just uses a wildcard.

[nfischer]

Fixed by #78