Support variable substitutions in `bin_path_in_archive`

[my4ng]

This fixes #122 by allowing bin_path_in_archive to refer to variables that are only available at runtime, especially the release version, which may not be known when getting the latest release without additional queries. It uses {{ var }} syntax and supports bin, target and version.

Example:

myapp.tar/
 |------- windows-1.2.3-bin/
 |         |--- myapp  # <-- executable

Use "{{ target }}-{{ version }}-bin/{{ bin }}" for bin_path_in_archive.

Note:

The regex that matches the syntax accepts zero or more ASCII whitespace characters around the var. Hence both {{var}} and {{ var }} would work.

[jaemk]

Thanks @my4ng !