Add fixer for json: python's json.tool (#4847)

Resolves #4314.

Add a fixer that's built into python for json formatting. Include a
couple arguments in docs to make these features more discoverable.

Uses stdin-based fixing so you don't need to save the file to fix.

autoload/ale/fix/registry.vim

@@ -477,6 +477,11 @@ let s:default_registry = {
 \       'suggested_filetypes': ['json'],
 \       'description': 'Fix JSON files with jq.',
 \   },
+\   'json_pytool': {
+\       'function': 'ale#fixers#json_pytool#Fix',
+\       'suggested_filetypes': ['json'],
+\       'description': "Fix JSON files with python's built-in json.tool module.",
+\   },
 \   'protolint': {
 \       'function': 'ale#fixers#protolint#Fix',
 \       'suggested_filetypes': ['proto'],

autoload/ale/fixers/json_pytool.vim

@@ -0,0 +1,20 @@
+" Author: idbrii
+" Description: json formatter as ALE fixer using python's json.tool
+
+call ale#Set('json_pytool_executable', 'python')
+call ale#Set('json_pytool_options', '')
+call ale#Set('json_pytool_use_global', get(g:, 'ale_use_global_executables', 0))
+
+function! ale#fixers#json_pytool#GetExecutable(buffer) abort
+    return ale#path#FindExecutable(a:buffer, 'json_pytool', ['python'])
+endfunction
+
+function! ale#fixers#json_pytool#Fix(buffer) abort
+    let l:executable = ale#Escape(ale#fixers#json_pytool#GetExecutable(a:buffer))
+    let l:opts = ale#Var(a:buffer, 'json_pytool_options')
+    let l:command = printf('%s -m json.tool %s -', l:executable, l:opts)
+
+    return {
+    \   'command': l:command
+    \ }
+endfunction