Allow reltool target_dir to be constructed on the fly

[uwiger]

The version info in reltool.config can already be expanded by rebar.
This patch makes it possible to use release name and version info to
dynamically construct a target_dir. The patterns $(name) and $(vsn)
are automatically expanded to the corresponding values in RelToolConfig.

For example:

{target_dir, "$(name)/$(vsn)"}

would result in a target directory like "rel/mynode/2.2.1".

[uwiger]

Superceded by the support for .script config files that has just been merged into master.