Removed syntastic and replaced it with ale

Read more here:
https://github.com/w0rp/ale

sources_non_forked/ale/ale_linters/nix/nix.vim

@@ -0,0 +1,26 @@
+" Author: Alistair Bill <@alibabzo>
+" Description: nix-instantiate linter for nix files
+
+function! ale_linters#nix#nix#Handle(buffer, lines) abort
+    let l:pattern = '^\(.\+\): \(.\+\), at .*:\(\d\+\):\(\d\+\)$'
+    let l:output = []
+
+    for l:match in ale#util#GetMatches(a:lines, l:pattern)
+        call add(l:output, {
+        \   'lnum': l:match[3] + 0,
+        \   'col': l:match[4] + 0,
+        \   'text': l:match[1] . ': ' . l:match[2],
+        \   'type': l:match[1] =~# '^error' ? 'E' : 'W',
+        \})
+    endfor
+
+    return l:output
+endfunction
+
+call ale#linter#Define('nix', {
+\   'name': 'nix',
+\   'output_stream': 'stderr',
+\   'executable': 'nix-instantiate',
+\   'command': 'nix-instantiate --parse -',
+\   'callback': 'ale_linters#nix#nix#Handle',
+\})