Suppress warning about .eslintignore'd file (#836)

* Suppress warning about .eslintignore'd file

* Fix slightly ironic lint error

* Lock error suppression behind a variable; add docs and tests

autoload/ale/handlers/eslint.vim

@@ -4,6 +4,7 @@
 call ale#Set('javascript_eslint_options', '')
 call ale#Set('javascript_eslint_executable', 'eslint')
 call ale#Set('javascript_eslint_use_global', 0)
+call ale#Set('javascript_eslint_suppress_eslintignore', 0)
 
 function! ale#handlers#eslint#GetExecutable(buffer) abort
     return ale#node#FindExecutable(a:buffer, 'javascript_eslint', [
@@ -82,6 +83,12 @@ function! ale#handlers#eslint#Handle(buffer, lines) abort
         let l:type = 'Error'
         let l:text = l:match[3]
 
+        if ale#Var(a:buffer, 'javascript_eslint_suppress_eslintignore')
+            if l:text is# 'File ignored because of a matching ignore pattern. Use "--no-ignore" to override.'
+                continue
+            endif
+        endif
+
         " Take the error type from the output if available.
         if !empty(l:match[4])
             let l:type = split(l:match[4], '/')[0]