Split {format} and allow use of {binary-ext} in pkg-url

[passcod]

This from feedback in #19:

wrt. bin-dir and bin-path, this appears to be a typo / should all be called bin-dir

This is only a readme fix afaict, I changed all occurences of bin-path in there to bin-dir.

wrt. format, those are actually two (unfortunately named) different concepts, the first refers to the archive format (eg. .tgz), the second to the binary format (which needs a .exe appended for windows).

This introduces two new substitutions:

• binary-ext is the old "format in bin-dir"
• archive-format is the old "format in pkg-url"

Contents are unchanged: binary-ext includes the dot, archive-format doesn't. That makes it easy to upgrade and also personally I slightly prefer it that way.

The old contextual format is still available, "soft deprecated": it will be accepted silently so everything will work, but all documentation will use the new syntax. In the future we could move to a "hard deprecated" model where installing a package that uses format will warn the user / tell them to report that to the maintainer. I don't think we'll ever really be able to remove it but that should be good enough.

A cool new feature is that binary-ext is now usable in pkg-url, which will be useful for raw binary downloads:

pkg_url = "{ repo }/releases/download/v{ version }/{ name }-v{ version }-{ target }{ binary-ext }"

I've also added a bunch of tests to GhCrateMeta around the templating for pkg-url.

[ryankurte]

looks great!