Feature Request / Possible bug: Support nested variables enclosed by raw string

I have a weird issue where envsubst is not working as expected.

When I add only a single a default variable to the right side of the experssion it will work similarly as a default POSIX shell. But when I add additional text to the default it start to do some strange behaviour.

I'll provide some examples, I believe that will be the best description.

sh use case:
``` bash
docker run --rm -it debian:buster-slim sh
# export VAR1="shouldevaluate"
# export EMPTY_VAR=""
# export MAIN_VAR="${EMPTY_VAR:-$VAR1}"
# echo $MAIN_VAR
shouldevaluate
#
#
# export MAIN_VAR="${EMPTY_VAR:-asdqwe$VAR1}"
# echo $MAIN_VAR
asdqweshouldevaluate
#
#
# export MAIN_VAR="${EMPTY_VAR:-asdqwe $VAR1 qweasd}"
# echo $MAIN_VAR
asdqwe shouldevaluate qweasd
#
#
# export MAIN_VAR="${EMPTY_VAR:-asdqwe${VAR1}qweasd}"
# echo $MAIN_VAR
asdqweshouldevaluateqweasd
```

And this happens when I try to use the same method in envsubst.
``` bash
# cat test.template
name = ${EMPTY_VAR:-$VAR1}

name = ${EMPTY_VAR:-asdqwe$VAR1}

name = ${EMPTY_VAR:-asdqwe $VAR1 qweasd}

name = ${EMPTY_VAR:-asdqwe${VAR1}qweasd}
#
#
# envsubst < test.template
name = shouldevaluate

name = asdqwe$VAR1

name = asdqwe $VAR1 qweasd

name = asdqwe${VAR1qweasd}
```