Variables should be templated out ahead of parsing rules

-- rules --
myattr = NEW
myeditor = ad

data is hello, world!
dst is $myeditor
attr add b=$myattr
attr add c=baz
plumb to edit

-- initial --
src: bash
dst: ad
wdir: /home/foo/bar
attrs: a=foo b=bar
ndata: 13
data: hello, world!
-- processed --
src: bash
dst: ad
wdir: /home/foo/bar
attrs: a=foo b=NEW c=baz
ndata: 13
data: hello, world!
