Make alex a file linter

alex does not find its configuration file when using temporary files for
input.

ale_linters/asciidoc/alex.vim

@@ -1,10 +1,11 @@
 " Author: Johannes Wienke <languitar@semipol.de>
 " Description: alex for asciidoc files
 
-call ale#linter#Define('asciidoc', {
+call ale#linter#Define('help', {
 \   'name': 'alex',
 \   'executable': 'alex',
-\   'command': 'alex %t -t',
+\   'command': 'alex %s -t',
 \   'output_stream': 'stderr',
 \   'callback': 'ale#handlers#alex#Handle',
+\   'lint_file': 1,
 \})