Allow external tool to be used for deletion (such as unlocker)

 - If a node has delete_command environment variable, it will be used
 	instead of the default functionality.

The format of the command is:
"path/to/executable" [options] %s

Unlocker, for example:
"C:\Program Files\Unlocker\unlocker.exe" %s /D /S
Deletion tool can be configured in the delete workspace settings.

Allows for expansion of variables so per-node configuration is still
available.