Add fish shell setup to `env` command and README (#10)

introduced a `--fish` flag to the env command, so fish users can add this line to their `config.fish` to set up fnm:

```fish
eval (fnm env --fish)
```

README.md

@@ -25,6 +25,12 @@
   eval `fnm env`
   ```
   
+  If you are using [fish shell](https://fishshell.com/), add this line to your `config.fish` file:
+  
+  ```fish
+  eval (fnm env --fish)
+  ```
+
 ## Future Plans
 - [ ] Add a simpler way of installing it (`curl | bash`?)
 - [ ] Feature: make versions complete the latest: `10` would infer the latest minor and patch versions of node 10. `10.1` would infer the latest patch version of node 10.1

executable/Env.re

@@ -1,9 +1,15 @@
 open Fnm;
 
-let run = () => {
+let run = isFishShell => {
+  if (isFishShell) {
+    Console.log(
+      Printf.sprintf("set PATH %s/bin $PATH", Directories.currentVersion),
+    );
+  } else {
     Console.log(
       Printf.sprintf("export PATH=%s/bin:$PATH", Directories.currentVersion),
     );
+  }
 
   Lwt.return();
 };

executable/FnmApp.re

@@ -5,7 +5,7 @@ module Commands = {
   let listRemote = () => Lwt_main.run(ListRemote.run());
   let listLocal = () => Lwt_main.run(ListLocal.run());
   let install = version => Lwt_main.run(Install.run(~version));
-  let env = () => Lwt_main.run(Env.run());
+  let env = isFishShell => Lwt_main.run(Env.run(isFishShell));
 };
 
 open Cmdliner;
@@ -88,8 +88,16 @@ let env = {
   let doc = "Show env configurations";
   let sdocs = Manpage.s_common_options;
   let man = help_secs;
+
+  let isFishShell = {
+    let doc = "Output an env configuration for fish shell.";
+    Arg.(
+      value & flag & info(["fish"], ~doc)
+    );
+  };
+
   (
-    Term.(const(Commands.env) $ const()),
+    Term.(const(Commands.env) $ isFishShell),
     Term.info("env", ~version, ~doc, ~exits=Term.default_exits, ~man, ~sdocs),
   );
 };