Retrieve environment variable in EDL

Currently environment variables doesn't availalbe within EDL scripts.
This force writing EDLs with hardcoded absolute paths making written scripts
not portable / less flexible.

This is suggested to implement this feature in some way. Possible solutions:
- parse input strings and replace substrings when special synax found (like
Unish ${MY_ENV_VAR} or NT %MY_ENV_VAR%);
- automatically fill EDL local variables list with evironment variables
(probably with special prefix like ENV_);
- add new keyword in syntax to retrieve environment variable (for example SET
VAR '=' ENV STR SEPARATOR { edl_set_var($2, edl_get_env($5)); edlstring_free
($2); edlstring_free($5);})