Support //boilerplate/clojure

While this is most likely incomplete or even erroneous, I'd like to start
supporting Clojure in this repository.

boilerplate/clojure/.gitignore

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+/.lein-repl-history
+/target
+/?
+/.nrepl-port

boilerplate/clojure/README.md

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+# Clojure Boilerplate
+
+This boilerplate uses `lein` to manage the project.
+
+## Files to change
+
+To use this boilerplate, run the following in a shell:
+
+```shell
+$ cp ~/briefcase/boilerplate/clojure path/to/new-project
+```
+
+After running the above command, change the following files to remove the
+placeholder values:
+
+- `README.md`: Change the title; change the description; drop "Files to change";
+  keep "Getting started"
+- `project.clj`: Change title
+- `src/project.clj`: Change `:doc`; drop `main/foo`
+
+## Getting started
+
+From a shell, run:
+
+```shell
+$ lein repl
+```
+
+From Emacs, navigate to a source code buffer and run:
+
+```
+M-x cider-jack-in
+```

boilerplate/clojure/dir-locals.nix

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+let
+  briefcase = import <briefcase> {};
+in briefcase.utils.nixBufferFromShell ./shell.nix

boilerplate/clojure/project.clj

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+(defproject boilerplate "0.0.1"
+  :dependencies [[org.clojure/clojure "1.8.0"]])

boilerplate/clojure/shell.nix

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+let
+  pkgs = import <nixpkgs> {};
+in pkgs.mkShell {
+  buildInputs = with pkgs; [
+    leiningen
+  ];
+}

boilerplate/clojure/src/main.clj

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+(ns ^{:doc "Top-level module."
+      :author "William Carroll"}
+    main)
+
+(declare main)
+
+(defn foo [a b]
+  (+ a b))