Make axoupdater an optional dependency (#109)

As in title; see comment in Cargo.toml as well as #107. Basically, with
this PR, users of verusfmt _as a library_ can use `default-features =
false` to obtain a stripped down version that doesn't inlcude
`axoupdater` as a dependency.

Closes #107

Cargo.toml

@@ -19,7 +19,7 @@ exclude = [
 ]
 
 [dependencies]
-axoupdater = { version = "0.7.2", features = ["blocking", "github_releases"], default-features = false }
+axoupdater = { version = "0.7.2", features = ["blocking", "github_releases"], default-features = false, optional = true }
 clap = { version = "4.3.11", features = ["derive"] }
 fs-err = "3.0.0"
 miette = { version = "7.2.0", features = ["fancy"] }
@@ -39,6 +39,13 @@ similar = { version = "2.2.1" }
 stacker = "0.1.15"
 glob-macro = { path = "glob-macro" }
 
+[features]
+# We enable the "optional" dependencies, that are only necessary for the binary,
+# as default-enabled features. If verusfmt is being used as a library, then that
+# program can turn default features off in order to obtain a more stripped-down
+# version of the library.
+default = ["axoupdater"]
+
 # Spend more time on initial compilation in exchange for faster runs
 [profile.dev.package.insta]
 opt-level = 3