Fix 2816 - Standard fix does not work.

The standard linter --fix fails if the file being input is not relative
to the project root (https://github.com/standard/standard/issues/1384).

This MR attempts to fix this by changing the command so the input file
is relative to the project root and the output is to a temporary file.

Preliminary tests with toy javascript projects seem to indicate this
works fine.

autoload/ale/fixers/standard.vim

@@ -27,7 +27,7 @@ function! ale#fixers#standard#Fix(buffer) abort
     return {
     \   'command': ale#node#Executable(a:buffer, l:executable)
     \   . (!empty(l:options) ? ' ' . l:options : '')
-    \       . ' --fix %t',
+    \       . ' --fix --stdin < %s > %t',
     \   'read_temporary_file': 1,
     \}
 endfunction