Add `hall run` command

Yo Alex,

I was looking for a way to not have to write autoreconf -vfi && ./configure && make && ./pre-inst-env frobnigator param1 param2 to run my cli app.

Executing hall run -- param… would be so much more handy haha

Bonus : if frobnigator.scm created using hall init could be filled with something like (display "hello world") that would be even better haha to quickly tell if everything can be run without error.