Allow custom filters for the jq fixer (#1980)

* Allow the jq filters to receive custom filters.
* Update documentation.

autoload/ale/fixers/jq.vim

@@ -1,5 +1,6 @@
 call ale#Set('json_jq_executable', 'jq')
 call ale#Set('json_jq_options', '')
+call ale#Set('json_jq_filters', '.')
 
 function! ale#fixers#jq#GetExecutable(buffer) abort
     return ale#Var(a:buffer, 'json_jq_executable')
@@ -7,9 +8,15 @@ endfunction
 
 function! ale#fixers#jq#Fix(buffer) abort
      let l:options = ale#Var(a:buffer, 'json_jq_options')
+     let l:filters = ale#Var(a:buffer, 'json_jq_filters')
+
+     if empty(l:filters)
+         return 0
+     endif
 
      return {
      \  'command': ale#Escape(ale#fixers#jq#GetExecutable(a:buffer))
-     \      . ' . ' . l:options,
+     \      . ' ' . l:filters . ' '
+     \      . l:options,
      \}
 endfunction