Add nixpkgs-fmt fixer.

This commit adds a fixer for the Nix language, nixpkgs-fmt
(https://github.com/nix-community/nixpkgs-fmt).

doc/ale-nix.txt

@@ -0,0 +1,24 @@
+===============================================================================
+ALE Nix Integration                                           *ale-nix-options*
+
+
+===============================================================================
+nixpkgs-fmt                                                 *ale-nix-nixpkgs-fmt*
+
+g:ale_nix_nixpkgsfmt_executable                *g:ale_nix_nixpkgsfmt_executable*
+                                               *b:ale_nix_nixpkgsfmt_executable*
+  Type: |String|
+  Default: `'nixpkgs-fmt'`
+
+  This variable sets executable used for nixpkgs-fmt.
+
+g:ale_nix_nixpkgsfmt_options                     *g:ale_nix_nixpkgsfmt_options*
+                                                 *b:ale_nix_nixpkgsfmt_options*
+  Type: |String|
+  Default: `''`
+
+  This variable can be set to pass additional options to the nixpkgs-fmt fixer.
+
+
+===============================================================================
+  vim:tw=78:ts=2:sts=2:sw=2:ft=help:norl:

doc/ale-supported-languages-and-tools.txt

@@ -281,6 +281,7 @@ Notes:
   * `nim check`!!
 * nix
   * `nix-instantiate`
+  * `nixpkgs-fmt`
 * nroff
   * `alex`!!
   * `proselint`

doc/ale.txt

@@ -2191,6 +2191,8 @@ documented in additional help files.
     mmc...................................|ale-mercury-mmc|
   nasm....................................|ale-nasm-options|
     nasm..................................|ale-nasm-nasm|
+  nix.....................................|ale-nix-options|
+    nixpkgs-fmt...........................|ale-nix-nixpkgs-fmt|
   nroff...................................|ale-nroff-options|
     write-good............................|ale-nroff-write-good|
   objc....................................|ale-objc-options|