repeating replacement of variables

[sassman]

This is a pre requisite for issue #18

A variable definition can contain a variable as well:

curlz r \ 
  --define hostname=httpbin.org \
  --define port=443 \
  --define 'host={{hostname}}:{{port}}' \
  'https://{{host}}/get'

Here hostname and port are used by host, therefore the variable value of host must be substituted twice, so that it is eventually equivalent to --define host=httpbin.org:443

Note: this should not happen only twice but as long as variables are being used inside of a defined variable.