Make it easier to export attributes

[TekWizely]

In order to actually use attributes in scripts, commands need to first convert them into environment variables, which often leads to verbose assignments like:

(assumes #55 merged)

export RUNFILE_DIR := ${.RUNFILE.DIR}

Perhaps we can make exporting attributes easier for the user:

Quick Export

A quick export could be as easy as:

##
# export .RUNFILE.DIR
#
main:
    echo $RUNFILE_DIR

ie. the quick export rules could be:

• Ignore the leading .
• Replace . with _

Export With Rename

We could also support an easy way to export with a different name:

##
# export .RUNFILE.DIR as RFD
#
main:
    echo $RFD

Support Variables Too?

The as syntax could also be used as a way to export standard variables with a different name.

Although I'm not sure how useful this would be?

Even if its not overly useful, we might want to support it just for symmetry with attributes