Adds .RUN and .RUNFILE attributes

[TekWizely]

Adds .RUN and .RUNFILE attributes to make it possible to invoke other commands / runfiles from within your command script.

Runfile

##
# Invokes hello
# EXPORT RUN := ${.RUN}
# EXPORT RUNFILE := ${.RUNFILE}
test:
    "${RUN}" -r "${RUNFILE}" hello
hello:
    echo "Hello, World"

output

$ run test

Hello, World

---

Fixes #20