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

test/eslint-test-files/eslintignore/ignored.js

@@ -0,0 +1 @@
+var foo = "bar";

test/test_eslint_suppress_eslintignore.vader

@@ -0,0 +1,38 @@
+Before:
+  Save g:ale_javascript_eslint_suppress_eslintignore
+
+  call ale#test#SetDirectory('/testplugin/test')
+
+  runtime ale_linters/javascript/eslint.vim
+
+After:
+  Restore
+
+  call ale#test#RestoreDirectory()
+  call ale#linter#Reset()
+
+Execute(eslint should warn about ignored file):
+  call ale#test#SetFilename('eslint-test-files/eslintignore/ignore.js')
+
+  AssertEqual
+  \ [{
+  \    'lnum': 0,
+  \    'col': 0,
+  \    'type': 'W',
+  \    'text': 'File ignored because of a matching ignore pattern. Use "--no-ignore" to override. [Warning]'
+  \ }],
+  \ ale#handlers#eslint#Handle(347, [
+  \  '/path/to/some/ignored.js:0:0: File ignored because of a matching ignore pattern. Use "--no-ignore" to override. [Warning]',
+  \ ])
+
+
+Execute(eslint should not warn about ignored file when configured):
+  let g:ale_javascript_eslint_suppress_eslintignore = 1
+
+  call ale#test#SetFilename('eslint-test-files/eslintignore/ignore.js')
+
+  AssertEqual
+  \ [],
+  \ ale#handlers#eslint#Handle(347, [
+  \  '/path/to/some/ignored.js:0:0: File ignored because of a matching ignore pattern. Use "--no-ignore" to override. [Warning]',
+  \ ])