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`.

src/lib.rs

@@ -17,10 +17,10 @@ pub const TARGET: &'static str = env!("TARGET");
 
 /// Default package path template (may be overridden in package Cargo.toml)
 pub const DEFAULT_PKG_URL: &'static str =
-    "{ repo }/releases/download/v{ version }/{ name }-{ target }-v{ version }.{ format }";
+    "{ repo }/releases/download/v{ version }/{ name }-{ target }-v{ version }.{ archive-format }";
 
 /// Default binary name template (may be overridden in package Cargo.toml)
-pub const DEFAULT_BIN_PATH: &'static str = "{ name }-{ target }-v{ version }/{ bin }{ format }";
+pub const DEFAULT_BIN_DIR: &'static str = "{ name }-{ target }-v{ version }/{ bin }{ binary-ext }";
 
 /// Binary format enumeration
 #[derive(
@@ -83,7 +83,7 @@ impl Default for PkgMeta {
         Self {
             pkg_url: DEFAULT_PKG_URL.to_string(),
             pkg_fmt: PkgFmt::default(),
-            bin_dir: DEFAULT_BIN_PATH.to_string(),
+            bin_dir: DEFAULT_BIN_DIR.to_string(),
             pub_key: None,
             overrides: HashMap::new(),
         }
@@ -165,7 +165,7 @@ mod test {
 
         assert_eq!(
             &meta.pkg_url,
-            "{ repo }/releases/download/v{ version }/{ name }-{ target }.{ format }"
+            "{ repo }/releases/download/v{ version }/{ name }-{ target }.{ archive-format }"
         );
 
         assert_eq!(