add autoflake fixer (#3779)

* first attempt

* added autoflake executable

* added Windows executable for appveyor

* delete unused files

* corrected wrong sorting

autoload/ale/fixers/autoflake.vim

@@ -0,0 +1,28 @@
+" Author: circld <circld1@gmail.com>
+" Description: Fixing files with autoflake.
+
+call ale#Set('python_autoflake_executable', 'autoflake')
+call ale#Set('python_autoflake_use_global', get(g:, 'ale_use_global_executables', 0))
+call ale#Set('python_autoflake_options', '')
+
+function! ale#fixers#autoflake#Fix(buffer) abort
+    let l:executable = ale#python#FindExecutable(
+    \   a:buffer,
+    \   'python_autoflake',
+    \   ['autoflake'],
+    \)
+
+    if !executable(l:executable)
+        return 0
+    endif
+
+    let l:options = ale#Var(a:buffer, 'python_autoflake_options')
+
+    return {
+    \   'command': ale#Escape(l:executable)
+    \       . (!empty(l:options) ? ' ' . l:options : '')
+    \       . ' --in-place '
+    \       . ' %t',
+    \   'read_temporary_file': 1,
+    \}
+endfunction