Split {format} and allow use of {binary-ext} in pkg-url (#95)

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:

```toml
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`.

Cargo.toml

@@ -10,8 +10,8 @@ license = "GPL-3.0"
 
 
 [package.metadata.binstall]
-pkg-url = "{ repo }/releases/download/v{ version }/{ name }-{ target }.{ format }"
-bin-dir = "{ bin }{ format }"
+pkg-url = "{ repo }/releases/download/v{ version }/{ name }-{ target }.{ archive-format }"
+bin-dir = "{ bin }{ binary-ext }"
 
 [package.metadata.binstall.overrides.x86_64-pc-windows-msvc]
 pkg-fmt = "zip"