Add support for custom `bin` folder to prepend to `PATH`

[cipriancraciun]

In a similar fashion how z-run detects the library sources, it should detect bin folders that should be prepended to the PATH.

This can be used to:

• remove the need to update PATH environment in a project;
• on OSX / BSD to override the various tools with GNU variants;

[cipriancraciun]

This was solved by declaring extra paths to prepend to the PATH variable like so:

&&== path some-folder

(Where some-folder is relative to the current source file directory.)