Allow using `c` in `output_dir`

Using the c template function inside the definition of output_dir currently creates an infinite loop, since process_template (which is used by c) needs to know the value of output_dir.

As a workaround, we can set some value for output_dir in the options, for example:

output_dir: '[% c("var/projectname", { output_dir => "." }) %]/release'

rbm could do something like that automatically so we don't need to add some output_dir option in that case.