Add Vale linter for AsciiDoc

Vale supports AsciiDoc. This patch adds a Vale linter for AsciiDoc
files, which is based on the existing Markdown linter.

ale_linters/asciidoc/vale.vim

@@ -0,0 +1,9 @@
+" Author: Jeff Kreeftmeijer https://github.com/jeffkreeftmeijer
+" Description: vale for AsciiDoc files
+
+call ale#linter#Define('asciidoc', {
+\   'name': 'vale',
+\   'executable': 'vale',
+\   'command': 'vale --output=line %t',
+\   'callback': 'ale#handlers#unix#HandleAsWarning',
+\})