Nix workflow is clunky

## Problem

To compile, right now I do:

```
$ nix-shell
```

And to recompile:

```
$ <Ctr-C to exit>
$ nix-shell # again
```

## Proposal

Similarly to the postgrest repo, a:

```
$ plmustache-watch plmustache-build
```

Would be ideal. Even just a `plmustache-build` command where exiting manually is not needed would be an improvement.