Make the name field in projects optional (#870)

Closes #858.

If a project is named `default.project.json`, it acts as an `init` file
and gains the name of the folder it's inside of. If it is named
something other than `default.project.json`, it gains the name of the
file with `.project.json` trimmed off. So e.g. `foo.project.json`
becomes `foo`.

src/project.rs

@@ -39,7 +39,7 @@ enum Error {
 #[serde(deny_unknown_fields, rename_all = "camelCase")]
 pub struct Project {
     /// The name of the top-level instance described by the project.
-    pub name: String,
+    pub name: Option<String>,
 
     /// The tree of instances described by this project. Projects always
     /// describe at least one instance.