Begin supporting run

I'd like to be able to just call `run file.py` and have a program DWIM. I'm
working on run as a step in this direction. Define a simple configuration that
maps file extensions to template strings where "$file" is replaced with the
argv[1].

It basically works but there are outstanding TODOs. See the README and source
code for more information.

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+# run
+
+Simplify the commands you call to run scripts on the command line.
+
+```shell
+> run path/to/file.py
+> run path/to/file.ts
+```
+
+## How?
+
+Define a run.json configuration mapping commands to filename extensions like
+so:
+```json
+{
+  ".ts": "npx ts-node $file",
+  ".py": "python3 $file"
+}
+```
+
+Then call `run path/to/some/file.ts` on the command line, and `npx ts-node
+file.ts` will run.
+
+## Installation
+
+Install `run` using Nix.
+
+```shell
+> nix-env -iA briefcase.run
+```